Impulsive detection techniques in free space optical communications

ABSTRACT

Systems and methods are described for transmitting information optically. For instance, a system may include an optical source configured to generate a beam of light. The system may include at least one modulator configured to encode data on the beam of light to produce an encoded beam of light/encoded plurality of pulses. The system may include a spectrally-equalizing amplifier configured to receive the encoded beam of light/encoded plurality of pulses from the at least one modulator and both amplify and filter the encoded beam of light/encoded plurality of pulses to produce a filtered beam of light/filtered plurality of pulses, thereby spectrally equalizing a gain applied to the encoded beam of light. In some cases, the system may slice the beam of slight, to ensure a detector has impulsive detection. In some cases, the system may include a temperature controller to shift a distribution curve of wavelengths of the optical source.

FIELD OF THE DISCLOSURE

The subject matter described herein relates to free-space optical (FSO)wireless transmission including optical communications, remote-sensing,laser ranging, power beaming, etc., and more particularly, to enhancedoptical transport efficiencies that can be realized for wavelengthpropagation using short coherence length sources for beam propagationthrough a variably refractive medium such as the Earth's atmosphere.

BACKGROUND

FSO communications have potential to greatly increase data throughput,decrease cost, and increase access for high-speed internet and othercommunications technologies. To date, however, FSO communication systemshave had limited operational success due to atmospheric interference,which reduces the distance over which data can be optically transmittedand introduces bit errors. Meanwhile, alternative communicationstechnologies, such as radiofrequency and microwave communications, facesignificant spectrum limitations and cannot be used to deliversufficient data to meet demand. Currently available optical systems arenot able to produce sufficiently accurate, reliable, and available datatransmission results that can reliably offload communications demandfrom these radiofrequency and microwave systems and improve datatransmission and access, nor can currently available optical systemstransmit data over long distances.

Superluminescent diodes (SLEDs) produce substantial noise in the form ofrandom power fluctuations and have historically been unsuitable for usein carrier-grade FSO communications.

Accordingly, there is a need for optical communication systems that canprovide highly reliable, highly available data transmission over longdistances. Further, there is a need for optical communication that canreliably transmit data over long distances, such as half a mile or more.

SUMMARY

The following description presents a simplified summary in order toprovide a basic understanding of some aspects described herein. Thissummary is not an extensive overview of the claimed subject matter. Itis intended to neither identify key or critical elements of the claimedsubject matter nor delineate the scope thereof.

In some embodiments, an optical communication system may be provided foroptically transmitting data through a variably refractive medium. Theoptical communication system may include: an optical source configuredto generate a beam of light, the optical source comprising a waveguidethat amplifies emitted light; a modulator configured to encode data onthe beam of light to produce an encoded beam of light; and aspectrally-equalizing amplifier configured to receive the encoded beamof light from the modulator and both amplify and filter the encoded beamof light to produce a filtered beam of light, wherein thespectrally-equalizing amplifier spectrally equalizes a gain applied tothe encoded beam of light, wherein the optical communication system isconfigured to transmit the filtered beam of light through a variablyrefractive medium to a detector having a photoreceiver, wherein thephotoreceiver is configured to extract the data from the filtered beamof light.

In some embodiments, an optical communication system may be provided foroptically transmitting data through a variably refractive medium. Theoptical communication system may include: an optical source configuredto generate a beam of light, the optical source comprising a waveguidethat amplifies emitted light; a modulator configured to encode data onthe beam of light to produce an encoded beam of light; aspectrally-equalizing amplifier configured to receive the encoded beamof light from the modulator and both amplify and filter the encoded beamof light to produce a filtered beam of light, wherein thespectrally-equalizing amplifier spectrally equalizes a gain applied tothe encoded beam of light. The optical communication system may beconfigured to transmit the filtered beam of light through a variablyrefractive medium to a detector having a photoreceiver, wherein thephotoreceiver is configured to extract the data from the filtered beamof light. The system may further include a temperature controller havinga thermometer and a heater/cooler, wherein the temperature controllermay be configured to: sense, using the thermometer, a temperature of theoptical source; determine a temperature adjustment based on thetemperature, wherein the temperature adjustment is configured to modifya distribution curve of wavelengths of the beam of light; and adjust,using the heater/cooler, the temperature based on the temperatureadjustment.

In some embodiments, an optical communication system may be provided foroptically transmitting data through a variably refractive medium. Theoptical communication system may include: an optical source configuredto generate a beam of light, wherein the optical source includes awaveguide that amplifies emitted light; a first modulator configured toslice the beam of light into a plurality of pulses; a pre-amplifierconfigured to receive the plurality of pulses from the first modulatorand amplify the plurality of pulses to produce pre-amplified pluralityof pulses, wherein the pre-amplified plurality of pulses has an averagepower that corresponds to an average power of the beam of light; asecond modulator configured to encode data on the pre-amplifiedplurality of pulses to produce an encoded plurality of pulses; and aspectrally-equalizing amplifier configured to receive the encodedplurality of pulses and both amplify and filter the encoded plurality ofpulses to produce a filtered plurality of pulses, wherein thespectrally-equalizing amplifier spectrally equalizes a gain applied tothe encoded plurality of pulses, wherein the optical communicationsystem is configured to transmit the filtered plurality of pulsesthrough a variably refractive medium to a detector having aphotoreceiver, wherein the photoreceiver is configured to extract thedata from the filtered plurality of pulses at a rate less than adetection response time threshold of the detector.

Further variations encompassed within the systems and methods aredescribed in the detailed description of the invention below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, show certain aspects of the subject matterdisclosed herein and, together with the descriptions, help explain someof the principles associated with the disclosed implementations.

FIG. 1 depicts an example of an optical communications platformincluding free-space coupling of a USPL source as an optical source fortransport to a remote optical receive terminal.

FIG. 2 depicts an example of an optical communications platformincluding fiber coupling of a USPL source as an optical source fortransport to a remote optical receive terminal.

FIG. 3 depicts an example of an optical communications platformincluding fiber coupling of a USPL source to an external modulator fortransport to a remote optical receive terminal.

FIG. 4 depicts an example of an optical communications platformincluding fiber coupling of a USPL source to an external modulatorthrough a fiber medium for transport to a remote optical receiveterminal.

FIG. 5 depicts an example of a transmitting and or receiving elements,which can be of a type from either the Hyperbolic Mirror FabricationTechniques or conventional Newtonian designs.

FIG. 6 depicts an example of an optical fiber amplifier elementidentified and used to increase enhancing optical transmit launch powerfor transport to a remote optical receive terminal.

FIG. 7 depicts an example of a USPL laser device that is fiber coupledto an external modulator for transport, in a point-to-pointconfiguration for transport to a remote optical receive terminal.

FIG. 8 depicts an example of a USPL laser device that is fiber coupledto an external modulator for transport, in a point-to-multi-pointconfiguration.

FIG. 9 depicts an example of use of USPL sources acting as tracking andalignment (pointing) beacon sources.

FIG. 10 depicts an example of a USPL laser sources polarizationmultiplexed onto a transmitted optical signal, to provide PolarizationMultiplex USP-FSO (PM-USP-FSO) functionality.

FIG. 11A and FIG. 11B respectively depict examples of USPL-FSOtransceivers utilized for use in line-of-sight and non-line-of-sightlasercom applications.

FIG. 12 depicts an example of light including light from the data signalpropagated forward being backscattered by interaction with air-borneparticulates that are the subject of investigation.

FIG. 13 depicts an example of USPL laser sources as optics receptiontechniques to improve detection sensitivity.

FIG. 14 depicts an example of a USPL-FSO transceiver utilized andoperated across the infrared wavelength range optionally including lightfrom the data signal as a range-finder and spotting apparatus for thepurposes of target identification.

FIG. 15 depicts an example of a USPL pulse multiplier device consistentwith implementations of the current subject matter.

FIG. 16 depicts another example of a device for generation of high pulserate USPL optical streams consistent with implementations of the currentsubject matter.

FIG. 17 depicts another example of an optical device to a generate aUSPL RZ data stream from a conventional transmission networking element.

FIG. 18 depicts an example of a implementing a USPL pulse multiplierdevice for generation of 10×TDM type signals system to give a 100 Gbpsoutput.

FIG. 19 depicts an example of a implementing another type of USPL pulsemultiplier device for extending the pulse repetition rate for use inhigh capacity networks.

FIG. 20 depicts an example of a implementing another type of USPL pulsemultiplier device for extending the pulse repetition rate for use inhigh capacity networks.

FIG. 21 depicts examples of active mode-locked linear fiber lasers withfeedback regenerative systems: fiber reflector (FR), wavelength-divisionmultiplexer (WDM), Erbium-doped fiber (EDF), optical coupler (OC),photo-detector (PD), phase-locked loop (PLL), and Mach-Zehnder Modulator(MZM).

FIG. 22 and FIG. 23 depict examples of passive mode-locked linear fiberlasers using a carbon nano-tubes saturable absorber: fiber reflector(FR), wavelength-division multiplexer (WDM), Erbium-doped fiber (EDF),optical coupler (OC), and saturable absorber (SA).

FIG. 24 depicts an example of a time-delay stabilization mechanism:optical coupler (OCin, OCout), photo-detector (PDin, PDout), high-passfilter (HPF), low-pass filter (LPF), phase-locked loop (PLL),phase-comparator (PC), frequency-divider (1/N), clock-data recoverysystem (CDR), piezoelectric actuator (PZ1 . . . PZN), summing op amp,for use in stabilizing the optical pulse to pulse relationship producedfrom the USPL source.

FIG. 25A and FIG. 25B respectively include a schematic diagram and agraph relating to an example of a controlling mechanism to stabilize theoutput frequency of TDM sources utilizing an idealized PZ actuator.

FIG. 26 depicts an example of a Time-Domain Multiplexing (TDM) where theTDM multiplexes a pulse train using parallel time delay channels, havingthe delay channels to be “consistent” relative to each another (Becausethe frequency of an output multiplexed pulse train is ideally asinsensitive as possible to environmental changes, a feedback loopcontrol system can correct the delay units for any fluctuations whichcompromise the stability of the output rep rate, and feedback can beprovides through interconnection to a Neural Network).

FIG. 27 depicts an example of use of fiber based collimators along withPiezoelectric transducers for controlling individual MFC circuits.

FIG. 28 depicts an example of timing of the TDM chip from the USPLmodulation source to provide a Terabit/second (or faster) with aMultiplier Photonic chip.

FIG. 29 depicts an example of timing of the TDM chip from the USPLmodulation source to provide a Terabit/second (or faster) with aMultiplier Photonic chip operating in a WDM configuration.

FIG. 30 depicts an example of construction of a computer assistedsystem, which can control the pulse width of an all-fiber mode-lockedlaser using recursive linear polarization adjustments with simultaneousstabilization of the cavity's repetition rate using a synchronousself-regenerative mechanism and can also offer tunability of therepetition rate, and pulse width.

FIG. 31 depicts an example of a modified pulse interleaving scheme, by apulse multiplication technique, in which the lower repetition rate pulsetrain of a well-characterized, well-mode locked laser can be coupledinto an integrated-optical directional coupler, where a well-determinedfraction of the pulse is tapped off and “re-circulated” in an opticalloop with an optical delay equal to the desired inter-pulse spacing inthe output pulse train, and re-coupled to the output of the directionalcoupler.

FIG. 32 is a process flow chart illustrating features of a methodconsistent with implementations of the current subject matter.

FIG. 33 is another process flow chart illustrating features of a methodconsistent with implementations of the current subject matter.

FIG. 34 is another process flow chart illustrating features of a methodconsistent with implementations of the current subject matter.

FIGS. 35A and 35B show exemplary nodes that can be used for transmittingand/or receiving information.

FIG. 36 shows an exemplary arrangement in which data is transmitted froma first communications network to a second communications network overan optical communication distance D using a transmit node and areceiving node.

FIG. 37 shows an exemplary beam traveling over an optical communicationdistance D, such as 1 mile, through a constant refractive medium.

FIG. 38 provides a diagrammatic representation of photons in a beamtraveling through a variably refractive medium.

FIG. 39 shows a diagrammatic representation of a pulse broadening as ittravels over an optical communication distance.

FIG. 40 shows an exemplary temporal distribution curve of ashort-duration pulse that traveled a substantial distance through avariably refractive medium and has been temporally broadened.

FIG. 41 shows a diagrammatic representation of light pulses arriving indetection windows of a photoreceiver.

FIG. 42 shows an example of test data received over a one-mile opticalcommunication distance.

FIG. 43 shows an exemplary ranging node that can be used to detectobjects or surfaces and determine positions of those objects relative tothe node.

FIG. 44 shows an exemplary optical communication system for transmittingdata through a medium.

FIG. 45 shows an exemplary optical source configured to generate anamplified spontaneous emission (ASE) output.

FIG. 46 shows an exemplary optical source configured to generate anoutput for use in a communication system.

FIG. 47 shows an exemplary fiber amplifier configured to amplify andfilter a beam of light.

FIGS. 48A-C show measurements of an exemplary continuous wave output.

FIGS. 48D-F show illustrative representations of the measurements shownin FIGS. 48A-C.

FIGS. 49A-C show measurements of an exemplary ASE output.

FIGS. 49D-F show illustrative representations of the measurements shownin FIGS. 49A-C.

FIG. 50 shows an exemplary optical communication system coupled to afiber optic gyroscope (FOG).

FIG. 51 shows an exemplary flow chart of the optical communicationsystem configured to transmit data through a medium.

FIG. 52 shows an exemplary system incorporating a spectrally-equalizingamplifier configured to equalize a gain of a beam of light andrespective distribution curves of wavelengths.

FIG. 53 shows an exemplary method of spectrally equalizing a gain of abeam of light using a spectrally-equalizing amplifier.

FIG. 54 shows an exemplary system for tuning a temperature of an opticalsource to optically transmit data through a refractive medium.

FIG. 55 shows an exemplary system with multiple sources used incombination to optically transmit data through a refractive medium.

FIG. 56A shows exemplary diagrams demonstrating how to align thewavelengths of the output of the source and the output of the amplifierto achieve a maximally broad output bandwidth.

FIG. 56B shows exemplary diagrams demonstrating a system configured totune a temperature of an optical source to account for a changed stateof the amplifier.

FIG. 57 shows an exemplary method for tuning a temperature of an opticalsource to optically transmit data through a refractive medium.

FIG. 58A shows an exemplary system for optically transmitting datathrough a variably refractive medium using pulses short enough wheredata can be extracted from a pulse at a rate less than a detectionresponse time.

FIG. 58B shows exemplary diagrams of power distributions of the beamgenerated by the optical source and of the chopped pulses.

FIG. 59 shows an exemplary method of for optically transmitting datathrough a variably refractive medium using pulses short enough wheredata can be extracted from a pulse at a rate less than a detectionresponse time.

DETAILED DESCRIPTION

While aspects of the subject matter of the present disclosure may beembodied in a variety of forms, the following description andaccompanying drawings are merely intended to disclose some of theseforms as specific examples of the subject matter. Accordingly, thesubject matter of this disclosure is not intended to be limited to theforms or embodiments so described and illustrated.

FIG. 1 illustrates an example of an optical communications platform 100configured to use an USPL source as an optical source for transport. Asshown in FIG. 1 , a USPL source 102 may be directly modulated by anexternal source element 104. Optical power from the USPL source 102 canbe coupled across free space 110 to a transmitting element 106,optionally by an optical telescope. The transmitting element 106 canoptionally include optical components formed by hyperbolic mirrorfabrication techniques, conventional Newtonian designs, or the like. Areciprocal receiving telescope at a receiver system can provide foroptical reception. Consistent with implementations of the currentsubject matter, each optical transport platform can be designed tooperate as a bi-directional unit. In other words, the transmittingelement 106 of the optical communications platform 100 can also functionas a receiving element. In general, unless otherwise explicitly stated,a transmitting element 106 as described can be considered to also befunctional as a receiving element and vice versa. An optical elementthat performs both transmission and receiving functions can be referredto herein as an optical transceiver.

FIG. 2 illustrates an example of an optical communications system 200that includes the optical communications platform 100 of FIG. 1 . Alsoshown in FIG. 2 is a second complementary receiving element 204, whichcan be a receiving telescope located at a remote distance from thetransmitting element 106. As noted above, both the transmitting element106 and the receiving element 204 can be bi-directional, and each canfunction as both a transmitting element 106 and a receiving element 204depending on the instantaneous direction of data transmission in theoptical communications system 200. This feature applies throughout thisdisclosure for transmitting and receiving elements unless otherwiseexplicitly stated. Either or both of the transmitting element 106 andthe receiving element 204 can be optical telescopes or other devices fortransmitting and receiving optical information.

FIG. 3 illustrates an example of an optical communications platform 300for using an USPL source 102 fiber coupled to an external modulator 302through a fiber medium 304 and connected to a transmitting element 106through an additional transmission medium 306, which can optionally be afiber medium, a free space connection, etc. The USPL source 102 can beexternally modulated by the external modulator 302 such that opticalpower from the USPL source 102 is fiber coupled to the transmittingelement 106 or handled via an equivalent optical telescope.

FIG. 4 illustrates an example of an optical communications system 400that includes the optical communications platform 300 of FIG. 3 . Alsoshown in FIG. 4 is a second complementary receiving telescope 204,which, as noted above in relation to FIG. 2 , can be a receivingtelescope located at a remote distance from the transmitting element106.

FIG. 5 illustrates an example of an optical communications architecture500. The architecture 500 of FIG. 5 may include the elements of FIG. 4and may further include a first communication network 502 connected to afirst optical communications platform 300. The receiving element 204 ispart of a second optical communications platform 504, which canoptionally include components analogous to those of the first opticalcommunications platform 300. A second communications network 506 can beconnected to the second optical communications platform 504 such thatthe data transmitted optically between the transmitting element 106 andthe receiving element 204 or are passed between the first and secondcommunications networks 502, 506, which can each include one or more ofoptical and electrical networking features.

FIG. 6 illustrates an example of an optical communications system 600.As part of an optical communications platform 602, an USPL source 102 isfiber coupled to an external modulator 302, for example through anoptical fiber 202 or other transmission medium. The light from the USPLsource 102 is propagated via a transmitting element 106 in a similarmanner as discussed above. An optical amplifier element 604, which canoptionally be an optical fiber amplifier element, can be used toincrease optical transmit launch power, and can optionally be disposedbetween the external modulator 302 and the transmitting element 106 andconnected to one or both via an additional transmission medium 306,which can optionally be a fiber medium, a free space connection, etc.Also shown in FIG. 6 is a second complementary receiving element 204located at a remote distance from the optical communications platform602. It will be readily understood that a second optical communicationsplatform 504 that includes the receiving element 204 can also include anoptical amplifier element 604. First and second communications networks502, 506 can be connected respectively to the two optical communicationsplatforms 602, 504.

FIG. 7 illustrates an example of an optical communications system 700.The optical communications platform 602 shown in FIG. 6 can be incommunication with a second optical communications platform 702, whichcan in this implementation include a receiving element 204 and anoptical preamplifier 704. Other components similar to those shown in theoptical communications platform 602 can also be included in the secondoptical communications platform 702, although they are not shown in FIG.7 . It will be understood that a bi-directional optical communicationsplatform can include both of an optical preamplifier 704 for amplifyinga received optical signal and an optical amplifier element 604 forboosting a transmitted optical signal.

Consistent with the implementation depicted in FIG. 7 and otherimplementations of the current subject matter, optical amplification(e.g. for either or both of an optical amplifier element 604 or anoptical preamplifier 704) be included for enhancing the optical budgetfor the data-link between the transmitting element 106 and the receivingelement 204 (and vice versa), for example using one or more of anerbium-doped fiber amplifier (EDFA), a high power erbium-ytterbium dopedfiber amplifier (Er/Yb-DFA), or equivalents, which can include but arenot limited to semiconductor-optical-amplifiers (SOA).

FIG. 8 illustrates an example of an optical communications system 800.The optical communications platform 602 shown in FIG. 6 can be incommunication with a second optical communications platform 802, whichcan in this implementation include a receiving element 204 and anoptical preamplifier 704 similar to those shown in FIG. 7 . As shown inFIG. 8 , the second optical communications platform 802 can furtherinclude optical receiver circuitry 804, which can receive amplified andelectrically recovered data received at the receiving element 204 andamplified by the optical preamplifier. A plurality of clock sources 806can interface to multiple remote multi-point network connections with aplurality of communications networks 810 as required. In a similarmanner, a complementary set of clock sources and multiple communicationnetworks can be operated in conjunction with the optical communicationsplatform 602 (e.g. in place of the single depicted communication network502 in FIG. 8 ).

FIG. 9 illustrates an example of an optical communications system 900.An optical communications platform 902, which can feature similarelements to those in the optical communications platform 602 firstdiscussed herein in reference to FIG. 6 , can also include an additionalUSPL source 904 acting as a tracking and alignment (pointing) beaconsource. A second optical communications platform 906 can also include anadditional USPL source 910 acting as a tracking and alignment (pointing)beacon source. The tracking and alignment (pointing) beacon sources 904,910 can optionally originate from available communications sources usedin data transport transmission, or can be provided by separate,dedicated USPL sources. In addition, each USPL beacon source 904, 910can include an in-band or out-of-band source, thereby allowing theadvantage of available optical amplification sources, or from dedicatedoptical amplification resources.

FIG. 10 illustrates an example of a FSO communication system 1000 thatincludes a dual polarization USPL-FSO optical data-link platform 1001 inwhich USPL sources are polarization multiplexed onto a transmittedoptical signal to thereby provide polarization multiplexed USP-FSO(PM-USP-FSO) functionality. Two USPL sources 102 and 1002 are fibercoupled to either directly modulated or externally modulated modulationcomponents 1004, 1006 respectively. Each respective modulated signal isoptically amplified by an optical amplifier component 1010, 1012followed by adjustment of optical polarization states using polarizationcomponents 1014, 1016. The polarization state signals are fiber coupledto a polarization dependent multiplexer (PDM) component 1020 forinterfacing to an optical launch platform component 1022, which can besimilar to the transmit element 106 discussed above. The PDM 1020multiplexes the light of differing polarization states into a singlepulse train for transmission via the optical launch platform component1022. An USPL optical beacon 904 can be included to provide capabilitiessimilar to those discussed above in reference to FIG. 9 , for example tooperate along or in conjunction with a second USPL optical beacon 906 ata receiving platform 1024, which can include a receiving element 204similar to those described above. As previously noted, the receivingelement 204 as well as other features and components of the receivingplatform 1024 can generally be capable of supporting transmissionfunctions such that a bi-directional link is established. A receivedsignal recovered by the receiving element 204 can provide an opticalsignal that is interfaced to an appropriate polarization dependentde-multiplexer 1026 capable of providing two signals for further opticalamplification using amplification elements 1030, 1032. Each opticalamplified signal as provided by the amplification elements 1030, 1032can be interfaced to an appropriate optical network 1034, 1036 fornetwork usage.

FIG. 11A shows an example of a system 1100 in which USPL-FSOtransceivers can be utilized for use in line-of-sight opticalcommunication (e.g. “lasercom”) applications, and FIG. 11B shows anexample of a system 1150 in which USPL-FSO transceivers can be utilizedfor use in non-line-of-sight laser communications applications. Anadvantage to some implementations of the current subject matter can berealized due to scattering of the optical signal sent from a transmitelement as the transmitted light passes through the atmosphere. Thisscattering can permit the use of non-line-of-sight communication. Inaddition, radios used in such communication systems can operate in thesolar-blind portion of the UV-C band, where light emits at a wavelengthof 200 to 280 nm. In this band, when solar radiation propagates throughthe environment, it is strongly attenuated by the Earth's atmosphere.This means that, as it gets closer to the ground, the amount ofbackground noise radiation drops dramatically, and low-powercommunications link operation is possible. On the other hand,environmental elements such as oxygen, ozone and water can weaken orinterrupt the communications broadcast, limiting usage to short-rangeapplications.

When UV waves spread throughout the atmosphere, they are typicallystrongly scattered into a variety of signal paths. Signal scattering isessential to UV systems operating in non-line-of-sight conditions, andthe communications performance can highly dependent on the transmissionbeam pointing and the receiver's field of view. A line-of-sightarrangement 1100 as shown in FIG. 11A can differ in bandwidth size froma non-line-of-sight arrangement 1150 as shown in FIG. 11B. Ultravioletcommunication can more strongly rely on a transmitter's beam positionand a receiver's field of view. As a result, refining of the pointingapex angle, for example by experimenting with supplementary equipment toenhance the UV-C signal, can be advantageous.

FIG. 12 illustrates an example of a remote sensing system 1200 in whichan USPL source 102 is fiber coupled by an optical fiber component 202 toan optical launch element 1202 capable of transmitting and receivingoptical signals. Some of the light propagated forward including thelight from data signal through the optical launch element 1202 isbackscattered by interaction with air-borne particulates that are thesubject of investigation. The optical backscattered signal is detectedthrough the optical launch element 1202 or a similar receive apertureand passed along for detection and spectrographic analysis throughdetection circuitry 1204 or the like in FIG. 12 . The signature ofparticulates within a target atmospheric region 1206 within which aninvestigation is made can be calibrated through known approaches, forexample using predetermined spectrographic calibration measurementsbased on one or more of ultraviolet spectroscopy, infrared spectroscopy,Raman spectroscopy, etc. Consistent with this implementation, an opticalsystem can be operated as a LIDAR instrument providing enhancedresolution and detection sensitivity performance, using USPL lasersources operating over a spectral range of interest. Adjustability ofspectral range can aid in evaluating and analyzing chemical constituentsin the atmosphere.

USPL-FSO transceivers can be utilized for remote sensing and detectionfor signatures of airborne elements using ionization or non-ionizationdetection techniques, utilizing optical transport terminals manufacturedthrough either the Hyperbolic Mirror Fabrication Techniques orconventional Newtonian designs that focus a received signal at one idealpoint. Also certain adaptations can be related to ionization probing ofremote regions include controllable ionization, which has been shown tooccur at these frequencies and an ionization process, which can befocused at distance to adjust depth of atmospheric penetrationespecially in weather and clouds.

FIG. 13 illustrates an example of use of USPL sources as well as opticalreception techniques to improve detection sensitivity. Researchers atthe National Institute of Standards and Technology (NIST), US, havebuilt a laser ranging system that can pinpoint multiple objects withnanometer precision over distances up to 100 km. The LIDAR (lightdetection and ranging) system could have applications from precisionmanufacturing on Earth to maintaining networks of satellites in perfectformation (Nature Photonics DOI: 10.1038/NPHOTON.2009.94). The NISTdevice uses two coherent broadband fiber-laser frequency combs.Frequency combs output a series of stable short pulses that also containa highly coherent carrier that extends across the pulse train. Thismeans a frequency comb can be employed to simultaneously make aninterferometric measurement as well as a time-of-flight measurement,thereby enhancing analytical capabilities for application specificsituations.

In the arrangement shown in FIG. 13 , two phase-locked frequency combs1301 and 1302 are used in a coherent linear optical samplingconfiguration, also known as a multi-heterodyne, meaning that onefrequency comb measures both distance paths, while the other frequencycomb provides distance information encoded in the light of the firstcomb. Pulses from one frequency comb 1301 can be launched out of thefiber and directed towards two glass plates, a reference 1303 and atarget 1304. The plates 1303 and 1304 can reflect a certain fraction(e.g. approximately 4%) of the pulse back down the fiber, effectivelycreating two new pulses. The time separation between the two pulses 1301can give the distance between the moveable target plate and referenceplates. A second frequency comb 1302 is tightly phase-locked with thefirst, but has a slightly different repetition rate. Due to thedifferent delay between consecutive pulses when the sources interfere,the second frequency comb can sample a slightly different part of thelight from the electric field of the first comb.

Using the technique described is reference to FIG. 13 , it is possibleto replace the two coherent broadband fiber-laser sources with twoappropriate USPL sources used within the scope of the configurationoutlined having each USPL source fiber coupled to dedicated free-spaceoptical telescope designs. By doing so, the overall efficiency, opticalranging and accuracy can be improved substantially.

In some embodiments, a native pulse repetition rate of a USPL lasersource and may be 50 MHz or less, which may be undesirably low foroptical data transmission, limiting the system to low data rateapplications of 50 Mbps or less. Accordingly, systems to increase USPLoperational rates are needed for providing solutions for data transportin excess of 50 Mbps.

FIG. 14 illustrates an example of a remote sensing system 1400 in whichan USPL source 102 is fiber coupled by an optical fiber component 202 toan optical launch element 1202 capable of transmitting and receivingoptical signals. Light propagated forward by the optical launch element1202 including light from the data signal is backscattered byinteraction with targets known and unknown that are the subject ofinvestigation within an atmospheric region 1206. The opticalbackscattered signal including light from the data signal is detectedthrough the optical launch element 1202 or a similar receive apertureand passed along for detection analysis through a detection circuitryand spectrographic analysis component 1402 in FIG. 14 . The signature ofparticulates within the region 1206 under investigation can becalibrated, for example where range-finding analysis can be performed. Asystem 1400 as in FIG. 14 can include a USPL-FSO transceiver utilizedand operated across the infrared wavelength range as a range-finder andspotting apparatus for the purposes of target identification andinterrogation applications. As used herein, the term “optical” includesat least visible, infrared, and near-infrared wavelengths.

FIG. 15 illustrates an optical pulse multiplier module 1500 that canincrease the repetition rate of the output from a USPL source 102. Anexemplary USPL may have a pulse width of 10-100 femto-seconds and arepetition rate of, for example, 50 MHz. The output from the USPL 102can be fed as an input 1502 into a USPL photonic chip pulse multipliermodule 1504. In this example, the photonic chip can contain a 20,000:1splitter element 1506 that splits the input into discrete lightelements. Each light element on the opposite side of the splitterelement 1506 contains the 50 MHz pulse train. Each light element thenpasses through a delay controller (either a fiber loop or lens array)1510, which delays the pulse train for that element in time, for exampleby a number of picoseconds. Successive light elements are therebydelayed by incremental picoseconds. All of these pulse trains with theirunique time delays are combined into a single pulse train in a fashionsimilar to time division multiplexing utilizing a 20,000:1 opticalcombiner element 1512. The required ratios of splitters and combinerscan be controlled to provide necessary optical designs for theapplication required. The final output 1514 is a pulse train of 10-100femto-second pulses with a repletion rate of 1 THz. This THz pulse traincan then be modulated by a 10 or 100 GigE signal, such as shown in FIG.28 , resulting in 100 femto-second pulses per bit for the 10 GigEsystem, and 10 femto-second pulses per bit for 100 GigE systems. Theapplication cited is not limited to specific data rates of 10 and 100Gbps, but can operate as required by the application underconsiderations. These numbers are just for illustration purposes.Implementations of the current subject matter can use any multiplierfactor to increase the repetition rate of the USPL via the photonic chippulse multiplier module 1504 to any arbitrary repetition rate. Otherexamples used in generation of enhanced USPL repetition rates areillustrated within this submission.

FIG. 16 depicts a system 1600 for generation, transmission, andreceiving of high pulse rate USPL optical streams. An optical chipmultiplexing module 1610, which can for example be similar to thatdiscussed in reference to FIG. 15 , can be used in this application. Inthis approach to achieve USPL pulse multiplication, a series of 10 GigErouter connections (10 GigE is not intended to be a limiting feature)described by signals 1601, 1602, 1603, 1604 (four signals are shown inFIG. 16 , but it will be understood that any number is within the scopeof the current subject matter) are interfaced to the optical chipmultiplexing module 1610. In operation, the optical chip multiplexingmodule 1610 can support full duplex (Tx and Rx) to connect with the 10GigE routers 1601, 1602, 1603, 1604. The optical chip multiplexingmodule 1610 can provide efficient modulation by a USPL signal 1685output from a USPL source 1690 for ingress optical signals 1601, 1602,1603, 1604. The optical chip multiplexing module 1610 can providecapabilities to modulate and multiplex these ingress optical signals.

At a remote receive site where a receiving device is positioned, allsignals sent via a transmitting element 1660 at the transmitting devicecan be recovered using an appropriate receiver element 1665. Acomplementary set of optical chip multiplexing module 1675 can providenecessary capabilities for demultiplexing the received data stream asshown by elements for delivery to a series of routers 1601′, 1602′,1603′, 1604′ (again, the depiction of four such routers is not intendedto be limiting). End-to-end network connectivity can be demonstratedthrough network end-point elements.

FIG. 17 depicts an example system 1700 in which an optical chip isinterconnected to a wavelength division multiplexing (WDM) system. WDMsystems have the advantage of not requiring timing or synchronization asneeded with a 10 GigE (or other speed) router 1701, since each 10 GigEsignal runs independent of other such signals on its own wavelength.Timing or synchronization of the TDM optical chip with 10 GigE routerscan be important in a TDM optical chip. A GbE switch 1701 can providethe necessary electrical RF signal 1705, from the switch 1701 tomodulate a USPL source 1702, either directly or by use of USPL a pulsemultiplier module previously detailed within this document. A typical RZoutput 1710 can be coupled into an external modulator 1720, which can bemodulated using a NRZ clock source for the switch 1701, therebyresulting in a RZ modulated spectrum 1730. The conversion process usingreadily available equipment can provide capabilities for introducingUSPL sources and their benefits into the terrestrial backhaul networkspectrum.

For the optical chip system to successfully bridge between two remote 10GigE switches, the chip may act like a simple piece of fiber. The timingof the TDM chip can therefore be driven by the 10 GigE switch 1701. Bothactively mode-locked USPLs (i.e. 40 GHz, 1 picosecond pulse width) andpassively mode-locked USPLs (i.e. 50 MHz, 100 femtosecond pulse width)can be driven by a RF timing signal.

FIG. 18 illustrates a device 1800 that can support another approach toprogression to a high pulse repetition data rate operation, such as forextremely high data rate operation in which optical chip design can beperformed using either fiber or free-space optics. A 50 MHz USPL source1801 may be interfaced to a series of optical delay controller elements1802, which can be designed using either fiber loops or offset lenses,to result in producing exactly a 10.313 Gbps RZ output stream, which isthe 10 GigE line rate (greater than 10 Gbps because of 64B/66Bencoding). A splitter element 1803 provides splitting functionality ofthe incoming optical signal train 1801 into (in this example) 206 paths,along with variable optical delay lines 1804. After sufficient delay isintroduced through design all signals are multiplexed together through acombiner element 1805. In so doing a series of optical signals eachidentical, and equally spaced between adjacent pulses form a continuumof pulses for modulation. Prior to entering an E-O modulator element1806, all optical ingress signals can be conditioned by pre-emphasistechniques, for example using typical optical amplification techniques,to result in a uniform power spectrum for each egress signal from thecombiner element 1805. The conditioned egress signals may then becoupled into the E-O modulator element 1806 and modulated with anavailable NRZ signal from the 10 GigE signal source element 1807. The 10GigE modulated output 1809 can interface to an EDFA and then into the TXof a FSO system (or a fiber optic system). The Rx side (after thedetector) can be fed directly into a 10 GigE switch as a modulated andamplified output 1810.

FIG. 19 illustrates another example of a device 1900 that can be usedfor USPL pulse multiplication consistent with implementations of thecurrent subject matter. Consistent with this approach, a 10×TDM systemis configured to give a 100 Gbps output. A TDM demux chip can be on thereceive side of a communication link to break up the individual 10 GigEsignals, and can include a reciprocal approach to the design shown inFIG. 19 .

As in FIG. 18 , a 50 MHz USPL source 1801 may be interfaced to a seriesof optical delay controller elements 1802, which can be designed usingeither fiber loops or offset lenses, to result in producing exactly a10.313 Gbps RZ output stream, which is the 10 GigE line rate (greaterthan 10 Gbps because of 64B/66B encoding). A splitter element 1803provides splitting functionality of the incoming optical signal train1801 into (in this example) 206 paths, along with variable optical delaylines 1804. After sufficient delay is introduced through design allsignals are multiplexed together through a combiner element 1805.Instead of a single modulator element 1806 as shown in FIG. 18 ,however, the 10.313 GHz RZ output 1901 from the combiner element 1805may be fed into a second splitter element 1910, which in this case canbe a 10× splitter, which splits the optical signal into ten parallelpaths. Other implementations of this design can support various splitratios as required by design. Optical paths out from second splitterelement 1910 are individually connected to specified optical delay lines1920. Each individual delayed path is connected to a dedicated opticalmodulator of a set of optical modulators 1930 modulated with anavailable NRZ signal from the 10×10 GigE signal source element 1931,resulting in a series of modulated optical signals 1935. An opticalcombiner identified 1940 provides a single optical pulse train 1950. Theseries of optical pulses in the single optical pulse train 1950 can beinterfaced to an appropriate optical amplifier for desired opticalconditioning for network use.

FIG. 20 illustrates another example of a device 2000 that can be usedfor USPL pulse multiplication consistent with implementations of thecurrent subject matter. A device 2000 as depicted can provide theability to achieve high USPL pulse repetition data rates for networkapplications by modulation of the low repetition rate intra-channelpulses. By applying direct modulation of each channel on the delaycontroller, creation of a modulation scheme, which is not constrained bythe current speed limitations from the electronics technology, can bebeneficially accomplished. Implementations of the current subject mattercan provide a mechanism to enhance the data transmission capacity of asystem, by separately modulating individual channels at the currentstandard electronic modulation speed (in the example of FIG. 20 at therate of 100×10 GigE signal input 2001) and time-multiplexing thechannels into a single frequency high rep rate pulse stream. In thisapproach, the current standard, which is limited by the speed ofelectro-optic modulators (40 Gbps), can be enhanced by approximately Norders of magnitude, where N is the number of channels of thetime-multiplexer. For example, a 100 channel TDM with each channelamplitude modulated at the current standard data rate can be able tooffer data rates at speeds of up to 4 Tbs. N can be limited by the widthof the optical pulse itself. In the limit that information is carried 1bit/pulse, the time slot occupied by 1 bit is the width of the pulseitself (in that sense, RZ system would converge to a NRZ). For example,in the scheme, a 40 fs pulse width laser with a 40 GHz rep rate is ableto carry information at a maximum rate of 25 Tbps. This approach can beused in a 40 Gbps-channel modulation scheme (i.e., 1 bit every 25 ps)and can correspond to a capacity of N˜625 channels in a singletransmission, which can be the number of 40 fs time intervals fitting ina 25 ps time interval. A significant advantage of this approach is theability to “optically enhance” an otherwise limited data capacitymodulation scheme, while still interfacing with the existing data ratelimited modulators. For example, an amplitude modulator based on aMach-Zehnder interferometer can be easily integrated in a TDM ICpackage, in that required is the ability to branch out the channel intotwo separate paths, add a tiny phase modulator (nonlinear crystal) inone of the paths, and combine the paths for interference.

FIG. 20 includes a USPL source 2010 coupled to a multi-port opticalsplitter element 2020. The number of optical ports identified need notbe limited to those described or shown herein. A series of optical delaylines 2030 provide required optical delays between each parallel pathfrom the multi-port optical splitter element 2020, and can be tailoredfor specific applications. The optical delay paths from the opticaldelay lines 2030 are summed together using an optical combiner element2035. The resulting combined optical data stream appearing throughelement 2040 represents a multiplicative enhancement in the pulserepetition rate of the original USPL source identified by element 2010.Further enhancement in pulse repetition rate is accomplished though theusage of element 2041, described by an optical splitter where theincoming signal 2040 is split into a series of paths not limited tothose identified by element 2041. By way of a second delay controller2045, optical delays may be introduced to each path within the device asidentified by the second set of optical delay paths 2042. Each parallelpath 2042 in turn is modulated by a modulating element 2044 with anavailable RF signal source element identified by the signal input 2001.An optical combiner element 2050 integrates all incoming signals onto asingle data stream 2060.

Optical pre-emphasis and de-emphasis techniques can be introduced withineach segment of elements described to custom tailor the optical spectrumfor a uniform or asymmetric optical power distribution. Pre- &de-emphasis can be accomplished using commonly used optical amplifierssuch as Er-Doped Optical amplifiers (EDFA).

FIG. 21 depicts an example of a system 2100 that includes a mode-lockedUSPL source 2101, which can be used to generate appropriately requiredclock and data streams for the application. Mode-locked lasers canrepresent a choice of high performance, high finesse source for clocksin digital communication systems. In this respect, mode-locked fiberlasers—in either linear or ring configuration—can make an attractivecandidate of choice, as they can achieve pulse widths on the USPL sourceregion and repetition rate as high as GHz. In addition to that, fibersoffer compactness, low cost, low sensitivity to thermal noise, lowjitter, no problems associated with diffraction or air dust pollution,just to name a few. In a communications scenario, the pulse width candetermine the available bandwidth of the system, and the repetition ratelimits the data rate. The pulse width can be determined by the intrinsiccharacteristics of the laser cavity—i.e. balancing of the overallgroup-velocity dispersion (GVD), and the choice of the saturableabsorber (in the case of a passive system)—or the bandwidth of an activeelement (in the case of an active mode-locked system). The repetitionrate of the pulse train is constrained by the length of the fiber. Forexample, in a linear laser, the fundamental mode frequency of the lasercan be expressed as:

$v_{osc} = \frac{c}{2n_{g}L}$where c is the speed of light in vacuum, n g is the average group index,and L is the length of the cavity. Therefore, a 10 cm long fiber lasercavity element 2110 with an average group index of 1.47 would have arepetition rate of 1 GHz. In strictly passive systems, mode-locking canbe achieved through the use of a saturable absorber. In an active laser,an amplitude modulator element 2150 can be inserted in the cavity toincrease the repetition rate of the laser (harmonic mode locking). Inorder to achieve high repetition rate clocks using mode-locked USPLsource, it is possible to use one or more of (i) an intra-cavityamplitude Mach-Zehnder modulator (MZM) 2150 as shown in FIG. 21 and (ii)a low threshold saturable absorber. These techniques, known as “harmonicmode-locking”, can be utilized within a fiber based plant distributionsystem or within a FSO system, for terrestrial, submarine or FSO systemeither in air, space or submarine applications.

Detailed within FIG. 21 is 980 nm pump element 2102 coupled to anoptical WDM device 2105. An erbium doped optical amplifier 2110 orequivalent can be used to create a non-linear environment to obtain amode-locked pulse train emission within a closed cavity establishedbetween two Faraday reflectors 2101 and 2160 on either end of theoptical USPL cavity. Operation of the device is capable of establishinga self-contained series of optical pulse in excess of 100 Gbps, andhighly synchronized in nature at the output port 2170 of the module. Inorder to achieve a high gain non-linear medium the EDFA 2110 can bespecially designed. A phase lock loop 2130 can provide advantageousstability in operation by maintaining a synchronized clock sourcethrough modulation of the signal through components 2120, 2130, 2150 ofthe self-contained high-repetition rate pulse generator. To achieve highrep rates in a laser that is limited by its dimensions (length in thecase of a linear laser and perimeter in the case of a ring laser), itcan be necessary to stimulate intra-cavity generation of the multiplesof the fundamental mode. In the active case, an amplitude modulatorinserted in the cavity modulates the loss of the system operating as a“threshold gating” device. For this approach to be successful, it can benecessary that the controlling signal to the modulator be referenced tothe oscillation of the laser itself to avoid the driving signal“forcing” an external frequency of oscillation on the laser. This can berealized by the introduction of a phase-lock-loop element 2130, or asynchronous oscillator circuit to track-and-lock onto the repetitionrate of the laser, and regenerate the signal. In the case of a PLL, theRF output can be set to a multiple of the input signal (much as thisdevice is used in cell phone technology), and the rep rate of the laserincreased. The signal can then be used for triggering of a pulsegenerator, or in conjunction with a low-pass filter. A MZ amplitudemodulator 2150 outside the laser cavity can be used to create On-OffKeying (OOK) modulation on the pulse train coming out of the mode-lockedlaser.

FIG. 22 shows a graphical depiction 2200 illustrating effects of a lossmodulation introduced to the input pulse train 2201 due to the presenceof the amplitude modulator 2205 with a controlling signal NRZ signal2210 made of a bit sequence as illustrated. The resulting signal at theoutput of the device 2220 represents an NRZ to RZ converter device foruse in telecommunications and scientific applications where theapplication may benefit from RZ data streams. A clock signal 2201(optical input) at a given pulse repetition rate will pass through themodulator 2205. At the same time, a controlling signal consisting of asequence of1's and 0's can be applied to the RF port of the modulatorelement 2215. When the modulator element 2215 is biased at minimumtransmission, in the absence of a controlling signal the lossexperienced by the optical signal can be at its maximum. In the presenceof the RF signal (1′5), the loss will drop to a minimum (OPEN GATE),thus working as an On-Off Keying modulation device. The pulse width ofthe output optical signal is typically much less than the time slotoccupied by a single bit of information (even less than a half clockperiod of a NRZ scheme) making this system genuinely RZ as identified byelement 2220.

FIG. 23 illustrates an example system 2300 for generation of highoptical harmonic USPL pulse streams having high pulse repetition rateusing a saturable absorber (SA) device 2330. The SA device 2330 can insome examples include carbon nanotubes. Passive mode-locked fiber lasersusing carbon nanotubes SA (CNT-SA) make another attractive option forhigh rep rate sources due to their ability to generate high harmonics ofthe fundamental rep rate. In the approach described, a closed,self-contained optical cavity is established, in which two Faradayreflectors 2301 and 2350 form the optical cavity. Although a high-powererbium doped fiber amplifier (EDFA) 2310 is shown in FIG. 23 , anyinverting medium producing a non-linear optical cavity can be used. Aseed laser 2315, such as for example a 980 nm pump laser as shown inFIG. 23 can be used in generating a high-repetition rate optical train.In particular, any suitable pump laser may be considered in terms ofoptical wavelength and pulse repetition rate required. The SA element2330 can be placed within the cavity to establish required optical pulsecharacteristics 2350 as required through design requirements.

FIG. 23 shows the schematics of an example of a laser that can be usedin one or more implementations of the current subject matter. Unlike theactive laser shown in FIG. 22 , here the MZ modulator can be replaced bythe SA element 2330. A technique similar to those described herein canbe utilized within a fiber based plant distribution system or within aFSO system, for terrestrial, submarine or FSO system either in air,space or submarine applications.

FIG. 24 illustrates an approach to providing time-domain multiplexing(TDM) where the TDM multiplexes a pulse train using parallel time delaychannels. In some instances, it can become important to manipulate thedelay channels such that they are “consistent” relative to each another.The frequency of the output multiplexed pulse train can ideally as muchas possible be insensitive to environmental changes. For that, aproposed feedback loop control system is design to correct the delayunits for any fluctuations which compromises the stability of the outputrep rate.

FIG. 24 shows a diagram of an example of a delay control system 2400.The control loop can be implemented in one of several ways consistentwith the current subject matter. FIG. 24 describes one possibility forillustration purposes. The input pulse train enters the TDM andmultiplexes into N paths, each with its own delay line. If the paths aremade of low “bending-loss” fiber waveguides, then each path can becoiled around a cylindrical piezoelectric actuator (PZ) of radius R. Theactuators generally expand in a radial direction as a result of acontrolling voltage (Vc). This expansion ΔR, which is linearlyproportional to Vc, causes a change in length of the fiber ΔL=2πNΔR,where N is the number of fiber turns around the PZ. For Terahertzmultiplexing, the delay between the pulses (and thus of PZ1) must be 1picosecond. This can require a change in length equals to 200 microns,which, for a one turn PZ actuator corresponds to a ΔR=32.5 microns. Mostcommercially available piezoelectric actuators are highly linear andoperate well within this range. The control mechanism can, therefore, bebased on several PZ actuators, each with a number of turns correspondingto multiples of the first delay, i.e. (32, 64, 96 microns, etc.), andcontrolled by a single voltage Vc. The controlling voltage is determinedby the feedback system, which compares the frequency of the outputsignal using a 1/N divider, with the frequency of the input signal,using a phase comparator (PC). The frequency of the “slow” input opticalsignal (represented by the waveform with TRT in FIG. 24 is converted toan RF signal using photo-detector PDin. In order to reduce the effectsof electronic jitter, a “differentiator” (or high pass filter) can beapplied to the RF signal as to steepen the leading edges of the pulses.A phase-locked loop is used to track-and-lock the signal, and toregenerate it into a 50% duty-cycle waveform. Likewise, in the outputside, the optical signal is picked-up by photo-detector PDout, high-passfiltered, and regenerated using the clock output port of aclock-and-data recovery system. The clock of the output signal, whichhas a frequency N times the frequency of the input signal, is send to anN times frequency divider before going to the phase comparator. From thephase comparator, a DC voltage level representing the mismatch betweenthe input and output signals (much as what is used in the architectureof PLL circuits) indicates the direction of correction for theactuators. A low-pass filter adds a time constant to the system toenhance its insensitivity to spurious noise.

A CDR can advantageously be used in the output, as opposed to a PLL suchthat the output signal may, or may not, be modulated. This system can bedesigned to work in both un-modulated, and “intra-TDM modulated” (i.e.one modulator at each delay path) schemes. However, this is a completelydeterministic approach to compensating for variations on the length ofthe delay lines. Ideally, and within a practical standpoint, the delaypaths should all be referenced to the same “thermal level” i.e. besensitive to the same thermal changes simultaneously. In the event thateach line senses different variation, this system would not be able tocorrect for that in real time.

In the alternative, a completely statistical approach can includesumming of op amp circuits (S1 . . . SN) to deliver the controllingvoltage to the actuators. Using such an approach, input voltages (V1 toVN) can be used to compensate for discrepancies in length between thelines, in a completely static sense, otherwise they can be used forinitial fine adjustments to the system. The approach typically must alsocompensate or at least take into account any bending loss requirementsof the fibers used. Some new fibers just coming out in the market mayhave a critical radius of only a few millimeters.

In the event that each path delay line senses different variation intemperature or experiences uncorrelated length changes due to spuriouslocalized noise, the previously described approach, as is, may sufferfrom difficulties in performing a real time correction. A more robustapproach operating in a completely statistical sense can be usedconsistent with some implementations of the current subject matter. Insuch an approach, summing op amp circuits (S1 . . . SN) can be used todeliver the controlling voltages to the actuators. In this case, theinput voltages (V1 to VN) can be used to compensate for discrepancies inlength between the delay lines in a completely statistical sense,otherwise they can only be useful for initial fine adjustments to thesystem (calibration).

Referring again to FIG. 24 , an incoming USPL source identified aselement 2401 is coupled to an optical coupler element 2403, such thatone leg of the coupler connects to an optical photodiode selected foroperation at the operational data rate of 2401. Using standardelectronic filtering techniques described by elements 2404, 2405, and2406 an electrical square wave representation of the incoming USPLsignal is extracted and identified by element 2407. The second opticalleg of coupler 2403 is interfaced into an appropriate optical splitterelement identified by 2410, where the incoming signal into 2410 is splitinto 206 parallel optical paths. Also illustrated are variable rateoptical delay lines established in parallel for each of the parallelbranches of the splitter element 2410. The parallel piezoelectricelements are identified by elements 242N and are controlledelectronically through feedback circuitry within the diagram. A controlvoltage identified by Vc is generated through a photodiode 2485 alongwith electronic circuitry elements 2480 and 2475. The clock-and-dataRecovery (CDR) element 2475 produces a clock source that is used incontrolling each of the PZ elements. Optical paths identified as 244Nare combined after a proper delay is introduced into each leg of element2410. The pulse multiplied USPL signal 2490 is thereby generated.

FIG. 25A shows a schematic of a fiber PZ actuator 2500, and FIG. 25Bshows a graph 2590 of radius vs. voltage for such an actuator. Together,these drawings illustrate operation of a PZ actuator for increasing thepulse repetition rate of an incoming USPL pulse train through inducedoptical delay. Although shown for use as an element for enhancing pulserepetition rate generation for USPL signals, the same technique can beused for other optical devices requiring or benefiting from opticaldelay. The basic structure for the device is a fiber based PZ actuator2501. When a voltage 2550 is applied to electrodes 2520 a voltageinduced stress results within the fiber, causing a time delay of theoptical signal traveling through the fiber. By varying applied voltage aperformance curve of optical delay vs. applied voltage is obtained asshown in the graph 2590 of FIG. 25B.

FIG. 26 shows a diagram illustrating features of an example statisticalcorrector 2600. The coarse correction controller 2640 shown in FIG. 26corresponds to the system described in the previous section, which cancorrect for length variations simultaneously picked up by all delaylines. As mentioned, these variations are expected to occur in a timescale much slower than the “intra delay line” spurious variations. Thislatter effect can manifest itself as a period-to-period jitterintroduced on the system. This type of jitter can be monitored using anRF Spectrum Analyzer (RFA), causing the rep rate line of the system todisplay “side lines” (or side bands), which are the result of theanalyzer beating together noisy frequencies resulting from uneven timeintervals between consecutive pulses. One such pattern can be processedusing an analog-to-digital converter (ADC) and saved as an array ofvalues, which can then be fed to a neural network (NN) machine. Neuralnetwork machines are known to possess excellent adaptabilitycharacteristics that allow them to essentially learn patterns fromoutside events by adapting to new set of input and outputs. A set ofinputs in this case can be generated from a set of “imperfectobservations”, i.e. “noisy” outputs of the TDM system as detected by theRFA and converted to digital arrays by the ADC ({f1, f2, . . . , fN},where fi is a frequency component picked up by the RFA). A set ofoutputs can be generated from the corrections ({V1, V2, . . . , VN},where Vt is a compensating input voltage to the summing op amp) requiredto rid the output frequency set from the undesired excess frequencynoise, which is due to the outside perturbations to the system. With asufficiently large number of {f,V} pairs, where f, V are frequency,voltage arrays, one can build an statistical set to train the NN machineto learn the underlying pattern associated with the presence of theintra-channel noise. These machines can be found commercially in an ICformat from several manufacturers, or implemented as software and usedin conjunction with a computer feedback control mechanism. A singlelayer Perceptron type neural network, or ADALINE (Adaptive Linear Neuronor later Adaptive Linear Element), should be sufficient to accomplishthe task.

Similar to the description provided above in relation to FIG. 24 , astatistical corrector element 2670 can include electronic circuitry thatis similar to or that provides similar functionality as the electricalcircuitry elements 2480 and 2475 and the photodiode 2485 of FIG. 24 .For the approach illustrated in FIG. 26 , a RF spectra analyzer 2695along with a Neural Network 2670 and a Coarse Correction Controllerelement 2640 are used to perform the requirement of optical delayintroduced into a parallel series of PZ elements 262N.

FIG. 27 illustrates concepts and capabilities of approaches consistentwith implementations of the current subject matter in which performance,accuracy, and resolution can be improved through replacement ofpiezoelectric disk (PZ) modules identified by elements 2795 and 272N,where compact micro fiber based collimators (MFC) 2795 encircled byceramic disks are used to obtain optical delay lines. Althoughillustrating a technique for increasing the native pulse repetition ratefor a USPL pulse train, the design illustrated is not limited to suchapplications but can be applied or extended to other needs within theoptical sector wherever optical delay is required. In so doing, a morecontrolled amount of temporal delay can be introduced within each MFCelement of the circuit. The improvement through the use of utilizing MFCelements can improve response, resolution, and the achievement ofreproducing in a rapid fashion required voltage responses in a massproduction means. The concept identified within FIG. 27 can beincorporated into precisely produced elements that can serve ascomplementary paired units for use in reducing USPL pulse-to-pulsejitter as well as for the purposes of data encryption needs.

With further reference to FIG. 27 , a USPL source 2701 having a certainpulse repetition rate is split into a preselected number of opticalpaths 271N (which can number other than 206) as identified by splitterelement 2705. An appropriately controlled delay 273N is introduced intoeach parallel leg of the split optical paths 271N using elementsdescribed by 2795 and 272N. The resulting delayed paths 274N are addedtogether through an optical combiner element 2760. The pulse multipliedUSPL signal 2780 results.

One potential disadvantage of some previously available TDM designs, inwhich fibers are “wrapped-around” the piezo actuators, is that themechanism must comply with the bending loss requirements of the fibersused. Some new fibers just coming out in the market have critical radiusof only a few millimeters. To correct for this issue, implementations ofthe current subject matter can use of micro-machined air-gap U-bracketsin lieu of the fiber-wrapped cylindrical piezo elements. FIG. 27illustrates this principle. In this approach, the piezoelectricactuators (PZ1, . . . PZN) can be replaced by air gap U-bracketstructures constructed using micro-fiber collimators (MFCs), andmicro-rings made of a piezoelectric material. In this case, however, thepiezoelectric actuator expands longitudinally, increasing (ordecreasing) the air gap distance between the collimators, in response tothe controlling voltages (V1, V2, . . . VN). As in the case of thecylindrical piezoelectric, a single voltage Vc can be use to drive allthe piezoelectric devices, provided that the gains of each channel (G1,G2, . . . GN) are adjusted accordingly to provide the correct expansionfor each line. Ideally, except for inherent biases to the system (i.e.intrinsic differences between op amps), the gain adjustments should beas G1, 2G1, 3G1, and so forth, in order to provide expansions, which aremultiples of the TRT/N. Another way of implementing such an approach canbe the use of multiple piezoelectric rings at the channels. In thatmanner, one can have channels with 1, 2, 3, N piezoelectric rings drivenby the same voltage with all amplifiers at the same gain.

FIG. 28 provides a conceptual presentation of an optical chip system2800 to successfully bridge between two remote 10 GigE switches.Ideally, such a connection can perform similarly to a simple piece offiber. The timing of the TDM chip can be driven by the 10 GigE switch.

In reference to FIG. 28 , a USPL source 2805 having a predeterminednative pulse repetition rate identified by 2806 connects to an opticalPulse multiplier chip 2807. Element 2807 is designed to convert theincoming pulse repetition rate signal 2806 into an appropriate level foroperation with high-speed network Ethernet switches as identified by2801. Switch 2801 provides a reference signal 2802 used to modulatesignal 2809 by way of a standard electro-optic modulator 2820 at thedata rate of interest. A resulting RZ optical signal is generated asshown in element 2840.

An alternative to having the timing run from the 10 GigE switch is tobuildup the USPL to a Terabit/second (or faster) with a multiplierphotonic chip, and then modulate this Terabit/second signal directlyfrom the 10 GigE switch. Each bit will have 100 or so pulses. Anadvantage of this approach can be the elimination of a need for separatetiming signals to be run from the switch to the USPL. The USPL viamultiplier chip just has to pump out the Terabit/second pulses. Anotheradvantage is that the output of the Multiplier Chip does not have to beexactly 10.313 or 103.12 Gbps. It just has to at a rate at about 1Terabit/second. Where each 10 GigE bit has 100 or 101 or 99 pulses, thislimitation is a non-issue. Another advantage is each bit will have many10 USPL, so the 10 GigE signal will have the atmospheric propagation(fog and scintillation) advantage. Another advantage can be realized atthe receiver end. It should be easier for a detector to detect a bit ifthat bit has 100 or so USPL pulses within that single bit. This couldresult in improved receiver sensitivity, and thus allow improved rangefor the FSO system. An additional advantage can be realized in thatupgrading to 100 GigE can be as simple as replacing the 10 GigE switchwith a 100 GigE switch. Each bit will have around 10 pulses in thiscase.

From a purely signal processing perspective this approach demonstratesan efficient way to send data and clock combined in a singletransmission stream. Much like a “sampling” of the bits using an opticalpulse stream, this approach has the advantage that the bit “size” isdetermined by the maximum number of pulses the it carries, thereforeestablishing a basis for counting bits as they arrive at the receivingend. In other words, if the bit unit has a time slot which can fit Npulses, the clock of the system can be established as “one new bit ofinformation” after every 5th.

A technique similar to those described herein can be utilized within afiber based plant distribution system or within a FSO system, forterrestrial, submarine or FSO system either in air, space or submarineapplications, and illustrates for the first time how the interconnectionfrom USPL sources to optical network elements is achieved for networkingapplications.

FIG. 29 shows a system 2900 that illustrates a conceptual networkextension for the design concept reflected within FIG. 28 . As multipleUSPL sources 2901, 2902, 2903 (it should be noted that while three areshown, any number is within the scope of the current subject matter),each modulated through dedicated optical switches and USPL laserMultiplier Chips circuits are configured in a WDM arrangement. Asdescribed in reference to FIG. 28 , electrical signals from eachEthernet switch can be used to modulate dedicated optical modulators2911, 2922, 2928 for each optical path. Optical power for each segmentof the system can be provided by optical amplification elements 2931,2932, 2933 for amplification purposes. Each amplified USPL path can thenbe interfaced to an appropriate optical combiner 2940 for transport to anetwork 2950, and can be either free space or fiber based as required.The output from the WDM module can then be configured to a transmittingelement 102 for FSO transport or into fiber plant equipment.

The technique described herein can be utilized within a fiber basedplant distribution system or within a FSO system, for terrestrial,submarine or FSO system either in; air, space or submarine applications,and illustrates for the first time how the interconnection from USPLsources to optical network elements is achieved for networkingapplications.

FIG. 30 shows the schematics of an experimental setup forimplementations of the current subject matter to include construction ofa computer assisted system to control the pulse width of an all-fibermode-locked laser using recursive linear polarization adjustments withsimultaneous stabilization of the cavity's repetition rate using asynchronous self-regenerative mechanism. The design can also offertune-ability of the repetition rate, and pulse width.

The fiber ring laser is represented by the inner blue loop, where allintra-cavity fiber branches are coded in blue, except for the positivehigh dispersion fiber outside the loop, which is part of the fibergrating compressor (coded in dark brown). The outside loops representthe feedback active systems.

FIG. 30 shows a diagram of a system 3000 illustrating features of anUSPL module providing control of pulse width and pulse repetition ratecontrol through mirrors (M1, M2), gratings (G1, G2), lengths (L1, L2),second-harmonic generator (SHG), photomultiplier tube (PMT), lock-inamplifier (LIA), data acquisition system (DAC), detector (DET),clock-extraction mechanism (CLK), frequency-to-voltage controller (FVC),high-voltage driver (HVD), reference signal (REF), pulse-generator(PGEN), amplitude modulator (AM), isolator (ISO), piezoelectric actuator(PZT), optical coupler (OC), polarizer (POL), and polarizationcontroller (PC) all serve to provide control of pulse repetition rateand pulse width control.

The passive mode-locking mechanism can be based on nonlinearpolarization rotation (NPR), which can be used in mode-locked fiberlasers. In this mechanism, weakly birefringent single mode fibers (SMF)can be used to create elliptically polarized light in a propagatingpulse. As the pulse travels along the fiber, it experiences a nonlineareffect, where an intensity dependent polarization rotation occurs. Bythe time the pulse reaches the polarization controller (PC) 3001 thepolarization state of the high intensity portion of the pulseexperiences more rotation than the lower intensity one. The controllercan perform the function of rotating the high intensity polarizationcomponent of the pulse, bringing its orientation as nearly aligned tothe axis of the polarizer (POL) as possible. Consequently, as the pulsepasses through the polarizer, its lower intensity components experiencemore attenuation than the high intensity components. The pulse comingout of the polarizer is, therefore, narrowed, and the entire processworks as a Fast-Saturable Absorber (FSA). This nonlinear effect works inconjunction with the Group-Velocity Dispersion (GVD) of the loop, and,after a number of round trips, a situation of stability occurs, andpassive mode-locking is achieved. The overall GVD of the optical loopcan be tailored to produce, within a margin of error, an specificdesired pulse width, by using different types of fibers (such as singlemode, dispersion shifted, polarization maintaining, etc. . . . ), andadding up their contributions to the average GVD of the laser.

An active control of the linear polarization rotation from the PC cangreatly improve the performance of the laser. This can be achieved usinga feedback system that tracks down the evolution of the pulse width.This system, represented by the outer loop in FIG. 1 , can be used tomaximize compression, and consequently, the average power of the pulse.A pulse coming out of the fiber ring laser through an OC is expected tohave a width on the order of a few picoseconds. An external pulsecompression scheme, which uses a fiber grating compressor, is used tonarrow the pulse to a sub 100 fsec range. This technique has beenextensively used in many reported experiments, leading to high energy,high power, USPL pulses. Here, the narrowed pulse is focused on aSecond-Harmonic Generator (SHG) crystal and detected using aPhoto-Multiplying Tube (PMT). The lock-in-amplifier (LIA) provides anoutput DC signal to a Data Acquisition Card (DAC). This signal followsvariations of the pulse width by tracking increases, or decreases, inthe pulses' peak power. A similar technique has been successfully usedin the past, except that, in that case, a Spatial Light Modulator (SLM)was used instead. Here, a programmable servo-mechanism directly controlsthe linear polarization rotation using actuators on the PC. With the DCsignal data provided by the DAC, a decision-making software (such as,but not limited to, LABVIEW or MATLAB SIMULINK) can be developed tocontrol the servo-mechanism, which in turn adjusts the angle of rotationof the input pulse relative to the polarizer's axis. These adjustments,performed by the actuators, are achieved using stress inducedbirefringence. For instance, if the pulse width decreases, the mechanismwill prompt the actuator to follow a certain direction of the linearangular rotation to compensate for that, and if the pulse widthincreases, it will act in the opposite direction, both aimed atmaximizing the average output power.

A self-regenerative feedback system synchronized to the repetition rateof the optical oscillation, and used as a driving signal to an amplitudemodulator (AM), can regulate the round trip time of the laser. In theactive system, the amplitude modulator acts as a threshold gating deviceby modulating the loss, synchronously with the round trip time. Thistechnique has can successfully stabilize mode-locked lasers in recentreports. A signal picked up from an optical coupler (OC) by aphoto-detector (DET) can be electronically locked and regenerated by aclock extraction mechanism (CLK) such as a Phase-Locked Loop or aSynchronous Oscillator. The regenerated signal triggers a PulseGenerator (PGen), which is then used to drive the modulator. In aperfectly synchronized scenario, the AM will “open” every time the pulsepasses through it, at each round trip time (TRT). Because the CLKfollows variations on TRT, the driving signal of the AM will varyaccordingly.

An outside reference signal (REF) can be used to tune the repetitionrate of the cavity. It can be compared to the recovered signal from theCLK using a mixer, and the output used to drive a Piezoelectric (PZT)system, which can regulate the length of the cavity. Such use of a PZTsystem to regulate the cavity's length is a well-known concept, andsimilar designs have already been successfully demonstratedexperimentally. Here a linear Frequency-to-Voltage Converter (FVC) maybe calibrated to provide an input signal to the PZT's High VoltageDriver (HVD). The PZT will adjust the length of the cavity to match therepetition rate of the REF signal. If, for instance the REF signalincreases its frequency, the output of the FVC will decrease, and sowill the HV drive level to the piezoelectric-cylinder, forcing it tocontract and, consequently increasing the repetition rate of the laser.The opposite occurs when the rep. rate of the reference decreases.

It is possible to have the width of the pulse tuned to a“transformed-limited” value using a pair of negative dispersiongratings. This chirped pulse compression technique is well established,and there has been reports of pulse compressions as narrow as 6 fs. Theidea is to have the grating pair pulse compressor mounted on a movingstage that translates along a line which sets the separation between thegratings. As the distance changes, so does the compression factor.

In an example of a data modulation scheme consistent withimplementations of the current subject matter, a passively mode lockedlaser can be used as the source of ultrafast pulses, which limits ourflexibility to change the data modulation rate. In order to scale up thedata rate of our system, we need to increase the base repetition rate ofour pulse source. Traditionally, the repetition rate of a passively modelocked laser has been increased by either shortening the laser cavitylength or by harmonic mode-locking of the laser. Both techniques causethe intra-cavity pulse peak power to decrease, resulting in longerpulse-widths and more unstable mode-locking.

One approach to solving this problem involves use of a modified pulseinterleaving scheme, by a technique which we call pulse multiplication.FIG. 31 illustrates this concept. The lower repetition rate pulse trainof a well-characterized, well-mode locked laser 3101 is coupled into anintegrated-optical directional coupler 3180, where a well-determinedfraction of the pulse is tapped off and “re-circulated” in an opticalloop with an optical delay 3150 equal to the desired inter-pulse spacingin the output pulse train, and re-coupled to the output of thedirectional coupler. For instance, to generate a 1 GHz pulse train froma 10 MHz pulse train, an optical delay of Ins is required, and to enablethe 100th pulse in the train to coincide with the input pulse from the10 MHz source, the optical delay might have to be precisely controlled.The optical delay loop includes optical gain 3120 to compensate forsignal attenuation, dispersion compensation 3160 to restore pulse-widthand active optical delay control 3150. Once the pulse multiplication hasoccurred, the output pulse train is OOK-modulated 3175 with a datastream 3182 to generated RZ signal 3190, and amplified in anerbium-doped fiber amplifier 3185 to bring the pulse energy up to thesame level as that of the input pulse train (or up to the desired outputpulse energy level).

One or more of the features described herein, whether taken alone or incombination, can be included in various aspects or implementations ofthe current subject matter. For example, in some aspects, an opticalwireless communication system can include at least one USPL lasersource, which can optionally include one or more of pico-second,nano-second, femto-second and atto-second type laser sources. An opticalwireless communication system can include USPL sources that can befiber-coupled or free-space coupled to an optical transport system, canbe modulated using one or more modulation techniques forpoint-to-multi-point communications system architectures, and/or canutilize optical transport terminals or telescopes manufactured throughone or more of hyperbolic mirror fabrication techniques, conventionalNewtonian mirror fabrication techniques, or other techniques that arefunctionally equivalent or similar. Aspheric optical designs can also oralternatively be used to minimize, reduce, etc. obscuration of areceived optical signal.

Free-space optical transport systems consistent with implementations ofthe current subject matter can utilize USPL laser designs that focus areceived signal at one ideal point. In some implementations onetelescope or other optical element for focusing and delivering light canbe considered as a transmitting element and a second telescope or otheroptical element for focusing and receiving light positioned remotelyfrom the first telescope or other optical element can function as areceiving element to create an optical data-link. Both opticalcommunication platforms can optionally include components necessary toprovide both transmit and receive functions, and can be referred to asUSPL optical transceivers. Either or both of the telescopes or otheroptical elements for focusing and delivering light can be coupled to atransmitting USPL source through either via optical fiber or by afree-space coupling to the transmitting element. Either or both of thetelescopes or other optical elements for focusing and receiving lightcan be coupled to a receive endpoint through either optical fiber or afree-space coupling to the optical receiver. A free-space optical (FSO)wireless communication system including one or more USPL sources can beused: within the framework of an optical communications network, inconjunction with the fiber-optic backhaul network (and can be usedtransparently within optical communications networks within an opticalcommunications network (and can be modulated using On-Off keying (OOK)Non-Return-to-Zero (NRZ), and Return-to-Zero (RZ) modulation techniques,within the 1550 nm optical communications band), within an opticalcommunications network (and can be modulated usingDifferential-Phase-Shift Keying (DPSK) modulation techniques), within anoptical communications network (and can be modulated using commonly usedmodulation techniques for point-to-point communications systemarchitectures using commonly used free-space optical transceiverterminals), within an optical communications network utilizing D-TEKdetection techniques, within a communications network for use inconjunction with Erbium-Doped Fiber Amplifiers (EDFA) as well as highpower Erbium-Ytterbium Doped Fiber Amplifiers (Er/Yb-DFA), within anoptical communications network (and can be modulated using commonly usedmodulation techniques for point-to-multi-point communications systemarchitectures), etc.

USPL technology can, in some aspects, be utilized as a beacon source toproviding optical tracking and beam steering for use in auto-trackingcapabilities and for maintaining terminal co-alignment during operation.The recovered clock and data extracted at the receive terminal can beused for multi-hop spans for use in extending network reach. The opticalnetwork can be provided with similar benefits in WDM configurations,thereby increasing the magnitude of effective optical bandwidth of thecarrier data link. USP laser sources can also or alternatively bepolarization multiplexed onto the transmitted optical signal to providepolarization multiplex USP-FSO (PM-USP-FSO) functionality. The recoveredclock and data extracted at the receive terminal can be used formulti-hop spans for use in extending network reach, and can include ageneric, large bandwidth range of operation for providing data-rateinvariant operation. An optical pre-amplifier or semi-conductor opticalamplifier (SOA) can be used prior to the optical receiver element and,alternatively or in combination with the recovered clock and dataextracted at the receive terminal, can be used for multi-hop spans foruse in extending network reach, having a generic, large bandwidth rangeof operation for providing data-rate invariant operation. Terminalco-alignment can be maintained during operation, such that significantimprovement in performance and terminal co-alignment can be realizedthrough the use of USPL technology, through the use of USPL data sourceas well as providing a improved approach to maintaining transceiveralignment through the use of USPL laser beacons.

USPL-FSO transceivers can be utilized in some aspects for performingremote-sensing and detection for signatures of airborne elements usingionization or non-ionization detection techniques, utilizing opticaltransport terminals manufactured through either the Hyperbolic MirrorFabrication Techniques or conventional Newtonian designs that focus areceived signal at one ideal point. USPL-FSO transceivers consistentwith implementations of the current subject matter can be utilized innon-line of sight lasercom applications. USPL-FSO transceiversconsistent with implementations of the current subject matter can allowadjustment of the distance at which the scattering effect (enabling NLOStechnique) takes place, reception techniques to improve detectionsensitivity using DTech detection schemes, and improved bandwidth viabroadband detectors including frequency combs. USPL-FSO transceiversconsistent with implementations of the current subject matter can beutilized in conjunction with Adaptive Optic (AO) Techniques forperforming incoming optical wave-front correction (AO-USPL-FSO).USPL-FSO transceivers consistent with implementations of the currentsubject matter can be utilized and operate across the infraredwavelength range. USPL-FSO transceivers consistent with implementationsof the current subject matter can be utilized in conjunction withoptical add-drop and optical multiplexing techniques, in bothsingle-mode as well as multi-mode fiber configurations. A USPL-FSOtransceiver consistent with implementations of the current subjectmatter can be utilized and operated across the infrared wavelength rangeas a range-finder and spotting apparatus for the purposes of targetidentification and interrogation applications.

In other aspects of the current subject matter, a series of switchednetwork connections, such as for example 10 GigE, 100 GigE, or the likeconnections can be connected from one point to another, either overfiber or free-space optics, for example via Time Division Multiplexing(TDM).

A mode-locked USPL source consistent with implementations of the currentsubject matter can be used to generate both clock and data streams.Mode-locked lasers can represent a choice of a high performance, highfinesse source for clocks in digital communication systems. In thisrespect, mode-locked fiber lasers—in either linear or ringconfiguration—can make an attractive candidate of choice, as they canachieve pulse widths of the USPL sources region and repetition rate ashigh as GHz.

High harmonic generation can be achieved using carbon nano-tubessaturable absorbers. Passive mode-locked fiber lasers using carbonnano-tubes saturable absorbers (CNT-SA) make an option for high rep ratesources due to their ability to readily generate high harmonics of thefundamental rep rate.

FSO can be used in terrestrial, space and undersea applications.

Conditional path lengths control from splitter to aperture can be animportant parameter. TDM multiplexes can be employed consistent withimplementations of the current subject matter to control the relativetemporal time delay between aperture-to-source paths. Each pulse traincan be controlled using parallel time delay channels. This technique canbe used to control conventional multiple-transmit FSO aperture systemsemploying WDM as well as TDM systems. USPL laser pulse-to-pulse spacingcan be maintained and controlled to precise temporal requirements forboth TDM and WDM systems. The techniques described can be used in TDMand WDM fiber based system. The use of TDM multiplexers as describedherein can be used implement unique encryption means onto thetransmitted optical signal. A complementary TDM multiplexer can beutilized to invert the incoming received signal, and thereby recover theunique signature of the pulse signals. A TDM multiplexer describedherein can be utilized to control WDM pulse character for the purpose ofWDM encryption. A TDM multiplexer can be used in conventional FSOsystems wherein multiple apertures connected to a common source signalare capable of having the temporal delay between pulses controlled tomaintain constant path lengths. A TDM multiplexer can be used for TDMfiber based and FSO based systems. A TDM multiplexer can be an enablingtechnology to control optical pulse train relationship for USPL sources.A TDM multiplexer can be used as an atmospheric link characterizationutility across an optical link through measurement of neural correctionfactor to get same pulse relational ship.

Any combination of PZ discs can be used in a transmitter and can have aninfinite number of encryption combinations for USPL based systems, bothfiber and FSO based. The timing can run from 10 GigE switches or theequivalent and to build up the USPL to a Terabit/second (or faster) ratewith a Multiplier Photonic chip, and this Terabit/second signal can bemodulated directly from the 10 GigE switch. While operating in a WDMconfiguration, an interface either to a fiber based system or to a FSOnetwork element can be included.

A system can accept an ultrafast optical pulse train and can generate atrain of optical pulses with pulse-width, spectral content, chirpcharacteristics identical to that of the input optical pulse, and with apulse repetition rate being an integral multiple of that of the inputpulse. This can be accomplished by tapping a fraction of the input pulsepower in a 2×2 optical coupler with an actively controllable opticalcoupling coefficient, re-circulating this tapped pulse over one roundtrip in an optical delay line provided with optical amplification,optical isolation, optical delay (path length) control, optical phaseand amplitude modulation, and compensation of temporal and spectralevolution experienced by the optical pulse in the optical delay line forthe purpose of minimizing temporal pulse width at the output of thedevice, and recombining this power with the 2×2 optical coupler.

Passive or active optical delay control can be used, as can optical gainutilizing rare-earth-doped optical fiber and/or rare-earth-dopedintegrated optical device and/or electrically- or optically-pumpedsemiconductor optical amplification. Dispersion compensation can beprovided using fiber-Bragg gratings and/or volume Bragg gratings.Wavelength division multiplexing data modulation of the pulse traversingthe delay line can be sued as can pulse code data modulation of thepulse traversing the delay line.

The tailoring of conventional USPL sources through synthesis of USPLsquare wave pulses can be accomplished utilizing micro-lithographicamplitude and phase mask technologies, for FSO applications. The abilityto adjust pulse widths using technology and similar approaches tocontrol and actively control pulse with this technology can improvepropagation efficiency through FSO transmission links, thereby improvingsystem availability and received optical power levels.

Active programmable pulse shapers can be used to actively control USPLpulse-width can include matching real-time atmospheric conditions tomaximize propagation through changing environments. One or more of thefollowing techniques can be used in FSO applications to adapt theoptical temporal spectrum using techniques: Fourier Transform Pulseshaping, Liquid Crystal Modular (LCM) Arrays, Liquid Crystal on Silicon(LCOS) Technology, Programmable Pulse Shaping using Acousto-opticmodulators (AOM), Acousto-optic Programmable Dispersive Filter (AOPDF),and Polarization Pulse Shaping.

FIG. 32 shows a process flow chart 3200 illustrating features of amethod, one or more of which can appear in implementations of thecurrent subject matter. At 3202, a beam of light pulses each having aduration of approximately 1 nanosecond or shorter is generated. At 3204,a modulation signal is applied to the beam to generate a modulatedoptical signal. The modulation signal carrying data for transmission toa remote receiving apparatus. The modulated optical signal is receivedat an optical transceiver within an optical communication platform at3206, and at 3210 the modulated optical signal is transmitted using theoptical transceiver for receipt by the second optical communicationapparatus

FIG. 33 shows another process flow chart 3300 illustrating features of amethod, one or more of which can appear in implementations of thecurrent subject matter. At 3302, a beam of light pulses each having aduration of approximately 1 nanosecond or shorter is generated, forexample using a USPL source. The beam of light pulses is transmitted at3304 toward a target atmospheric region via an optical transceiver. At3306, optical information received at the optical transceiver as aresult of optical backscattering of the beam of light pulses from one ormore objects in the target atmospheric region is analyzed.

FIG. 34 shows another process flow chart 3400 illustrating features of amethod, one or more of which can appear in implementations of thecurrent subject matter. At 3402, first and second beams comprising lightpulses are generated, for example by a USPL source. At 3404, a firstmodulation signal is applied to the first beam to generate a firstmodulated optical signal and a second modulation signal is applied tothe second beam to generate a second modulated optical signal. A firstpolarization state of the first modulated optical signal is adjusted at3406. Optionally, a second polarization states of the second modulatedoptical signal can also be adjusted. At 3410, the first modulatedoptical signal having the adjusted first polarization state ismultiplexed with the second modulated signal. At 3412, the multiplexedfirst modulated optical signal having the adjusted first polarizationstate with the second modulated signal is transmitted by an opticaltransceiver for receipt by a second optical communication apparatus.

FIGS. 35A and 35B show exemplary nodes that can be used for transmittingand/or receiving information. Transmit node 3510 and receiving node 3530may be communications platforms as described above, including withreference to FIGS. 1-9 . Additionally, while transmit node 3510 is shownwith components for generating and transmitting a data-bearing opticalsignal, and while receiving node 3530 is shown with components forreceiving and extracting data from an optical signal, these componentsmay be combined in a single node configured to both transmit and receiveoptical signals. In some embodiments, for example, a telescope 3522 mayact as both an aperture for transmitting and receiving optical signals.

FIG. 35A shows an exemplary transmit node 3510. In some embodiments,transmit node 3510 may include a source 3512. In some embodiments, thesource 3512 may be an USPL source, superluminescent diode, or othersource. In other embodiments, the source 3512 may be a continuous wavesource. Preferably, the source 3512 may be configured to generate a beamof light pulses, in which each pulse has a coherence length of less than400 microns. The coherence length of the source is determined as:

${L = {C\frac{\lambda^{2}}{\Delta\lambda}}},$where C is a shaping constant equal to ½, λ is the central wavelength ofthe pulse, and Δλ is the full width at half maximum (FWHM) spectralwidth of the pulse. In some embodiments, the coherence length may beless than 1 mm, less than 600 microns, less than 400 microns, less than200 microns, less than 100 microns, less than 50 microns, or less than10 microns. In embodiments where a continuous wave source is used, thesevalues may refer to the coherence length of the continuous wave beam,rather than that of the pulses.

In some embodiments, the source 3512 may have a central wavelength inthe infrared range. For example, the central wavelength of the source3512 may be between 1400 nm and 1700 nm. In some embodiments, the source3512 may be configured to output pulses at a repetition rate of at least50 MHz, 100 MHz, 200 MHz, 500 MHz, 800 MHz, 1 GHz, 1.25 GHz, 1.5 GHz, 2GHz, 5 GHz, or 10 GHz. The source 3512 may include (internally orexternally) a pulse multiplier, as generally described above, includingwith reference to FIGS. 15 and 18-20 . In some embodiments, the pulsewidth may be less than 10 ns, less than 1 ns, less than 500 ps, lessthan 300 ps, less than 100 ps, less than 50 ps, less than 10 ps, lessthan 1 ps, less than 700 fs, less than 500 fs, less than 300 fs, lessthan 200 fs, or less than 100 fs.

Transmit node 3510 may optionally include a splitter 3514. Splitter 3514may be configured to split pulses from source 3512 into a plurality ofseparated pulses having different wavelength bands. For example, a pulsehaving an original spectral width of 1500-1600 nm could be split intotwenty-five pulses, each having a respective spectral width of 4 nm from1500 nm to 1600 nm (e.g., 1500-1504 nm, 1504-1508 nm, 1508-1512 nm, andso on). Splitter 3514 may use any known beam-splitting mechanism. Eachof the plurality of separated pulses may have coherence lengths of lessthan 1 mm, less than 600 microns, less than 400 microns, less than 200microns, less than 100 microns, less than 50 microns, or less than 1micron.

Transmit node 3510 may include one or more modulators 3516. In someembodiments, each of the modulators 3516 may be a Mach-Zehnder Modulator(MZM). The modulators 3516 may receive a data signal indicating data tobe transmitted in an optical beam, and based on that data signal, mayencode the data into the pulses of the beam using on-off keying or othermodulation techniques. In some embodiments, the modulators 3516 mayallow pulses to pass to indicate a ‘1’ and may block or reduce theamplitude of a pulse to indicate a ‘0’ in a bit stream. In embodimentswhere the beam is split, each of a plurality of separated pulses may bedirected to a respective modulator 3516 of a plurality of modulators. Inother embodiments, each of the plurality of separated pulses may bemodulated by a single modulator 3516. For example, the separated pulsesmay be delayed and staggered in time relative to one another, and themodulator 3516 may encode data into each pulse at a higher repetitionrate than the pulse-generating repetition rate of the source. In a casewhere the source 3512 generates pulses at a rate of at least 1 GHz, forexample, the splitter may split each pulse into twenty-five or moreseparated pulses, which can be modulated by one or more modulators 3516to encode data at a rate of at least 25 Gbps. In some embodiments, thesource may generate pulses at a rate of at least 1 GHz, and the splittermay split each pulse into at least ten, at least twenty, at thirty, atleast forty, or at least fifty separated pulses, to produce data ratesof at least 10 Gbps, at least 20 Gbps, at least 30 Gpbs, at least 40Gpbs, or at least 50 Gbps. In some embodiments, the FWHM bandwidth ofthe source may be at least 100 nm, at least 150 nm, or at least 200 nm,which may allow pulses to be split into more separated pulses withoutreducing the coherence length of those pulses below the values describedbelow with respect to FIGS. 40 and 41 .

After being modulated, the pulses (optionally, the separated pulses inthe case where a splitter is used) may be passed to an optionalthresholding filter 3518. In some embodiments, the thresholding filtermay be a saturable absorber (or a different nonlinear device) thatattenuates weak pulses and transmits strong pulses. The thresholdingfilter 3518 may be configured to eliminate or substantially diminishpulses below a defined threshold, while allowing pulses above thatthreshold to pass. In some embodiments, modulator 3516 may significantlydiminish pulses where a “0” is intended to be transmitted, but it may beimperfect and some amount of optical energy may pass through, which,when amplified by amplifier 3520, could produce signals strong enough togenerate bit errors. By using a thresholding filter 3518, pulses thatare intended to be eliminated may be more fully eliminated, therebyimproving the system's data transmission accuracy.

The modulated pulses may be passed to an amplifier 3520, which mayincrease the magnitude of the pulses for transmission by telescope 3522(which may be, for example, an aperture and/or lens). In cases where asplitter is used, the separated pulses may be recombined using arecombiner (not shown) before or after being passed to amplifier 3520.

FIG. 35B shows an exemplary embodiment of a receiving node 3530, whichmay be configured to receive and extract data from an optical beamtransmitted by, e.g., a transmit node 3510. Receiving node 3530 mayinclude an aperture 3532, an optional splitter 3534, and one or morephotoreceivers 3536, which may have specific characteristics in relationto the source, as described in detail below. The photoreceivers 3536 mayinclude a photodiode and processing circuitry. In some embodiments, thephotoreceivers 3536 may be, for example, an avalanche photodiode. Insome embodiments, the processing circuitry of a photoreceiver maydetermine whether received light in a detection window exceeds adetection threshold and output bit data (e.g., a ‘0’ or ‘1’) for thatwindow based on the result of that determination. Receiving node 3530may be an optical communications platform as described above. In someembodiments, the components of transmit node 3510 and receiving node3530 may be included in a single transceiver node.

Aperture 3532 may be configured to receive an optical signal, such as anoptical beam transmitted by a transmit node 3510 as described in FIG.35A. In some embodiments, the light received at aperture 3532 may passthrough a filter that screens wavelengths of light that are not near thecenter wavelength of the source. For example, the source in the transmitnode may have a center wavelength between 1500 nm and 1700 nm, and thefilter at the receiving node 3530 may block or reduce light outside ofthe source band. For example, the filter may reduce a magnitude of lightbelow 1500 nm. Optionally, the filter may additionally block longerwavelengths of light, or the threshold may be set at lower wavelengths,such as at 1480 nm or 1460 nm. Optionally, receive node 3530 may includea splitter 3534, which may split pulses in a received beam into aplurality of separated pulses of different wavelength bands. In a casewhere the pulses are split and separately modulated at the transmit node3510, the pulses may be split into the same wavelength bands by thesplitter 3534 in the receive node. The pulses (combined pulses orseparated pulses, in the case where a splitter is used) may then beprocessed by one or more photoreceivers 3536. In embodiments where apulse is split into a plurality of separated pulses, each pulse may bedirected to a respective photoreceiver, which may be configured todetermine whether an “on” or “off” signal was transmitted in a givendetection window. In some embodiments, encoding modalities other thanon-off keying may be used, such as frequency modulation. Additionaldetail regarding photoreceivers 3536 is provided below with respect toFIG. 41 .

FIG. 36 shows an exemplary arrangement in which data is transmitted froma first communications network 3542 to a second communications network3544 over an optical communication distance D using a transmit node 3510and a receiving node 3530, such as those described above with respect toFIGS. 35A-35B. Data may be received from optical communications network3542 encoded into an optical beam and transmitted across opticalcommunications distance D using transmit node 3510. Receiving node 3530may receive the optical beam, extract the transmitted data, and pass thedata to communications network 3544. In some embodiments, data fromcommunications 3544 may also be transmitted from node 3530 back to node3510, which may pass that data to communications network 3542 to enabletwo-way communication. In some embodiments, optical communicationdistance may be at least miles, at least 1 mile, at least 2 miles, atleast 3 miles, at least 5 miles, at least 7 miles, at least 10 miles, orat least 20 miles.

FIG. 37 shows an exemplary beam traveling over an optical communicationdistance D, such as 1 mile, through a perfectly uniform refractive indexmedium. Even in a medium of perfectly constant index of refraction, thebeam will spread naturally due to diffraction, however the beam remainsthe same shape and simply expands by an amount that is proportional tothe propagation distance, and there are no beam scintillation effects ina uniform index of refraction medium.

FIG. 38 provides a diagrammatic representation of photons in a beamtraveling through a variably refractive medium. The atmosphere hasfluctuations in temperature, density, pressure, humidity, aerosols,wind, convection, and other parameters, which causes a refractive indexof the atmosphere to vary. As an optical beam travels through theatmosphere or other variably refractive medium such as water, photonswithin the beam may be refracted slightly differently than otherphotons. As shown in FIG. 38 , different ray paths within the beam maybe refracted differently due to variations in the refractive index inthe variably refractive medium. As a result, in a system such as thatshown in FIG. 35 where a free space optical beam is transmitted over asufficiently large optical communication distance D and received at areceiving node, different photons within a single pulse may take pathsof different lengths to reach the receiving node and may arrive atdifferent times. These differences in path length, and the time requiredfor a photon to travel these distances, can produce coherentinterference and diminish signal quality in a free space opticalcommunications system if the time delays are less than the coherencelength of the source. Solutions for this problem are described herein,including with reference to FIGS. 40 and 41 and as applied within asystem such as those shown in FIGS. 35A, 35B, and 36 .

In addition to variance in path length, photons in a pulse may travel atvariable speeds to due to variations in atmospheric conditions,including humidity, temperature, and density. Because different photonsin a pulse travel though slightly different atmospheric conditions, thephotons may travel at different speeds and arrive at different times.Additionally, different wavelengths of light within a pulse may travelat different speeds, which can further broaden a pulse as it travelsthrough a variably refractive medium.

FIG. 39 shows a diagrammatic representation of a pulse as launched by atransmitter and as received by a photoreceiver. As shown in FIG. 39 ,the pulse may have a 90 femtosecond pulse width when it is transmittedby a transmit node. The pulse may then travel over an opticaltransmission distance where it may be received by a photoreceptor havinga detection window 4020 of a defined duration, such as 500 picoseconds.When the pulse is received by the photoreceiver, its received pulsewidthmay be broadened by passing through the variably refractive medium, asdescribed above with respect to FIGS. 37-38 . Due to variance in pathlengths traveled by the beams and variance in atmospheric conditionsthrough which the beams travel, different photons may arrive at thedetector at different times according to a distribution curve, which mayhave a temporal duration that is longer than the pulse duration atlaunch. The amount of broadening can vary depending on the length of theoptical communication distance and atmospheric conditions, includinghumidity, temperature, density, and the presence of aerosols such asfog. This broadening can be the order of picoseconds or more in someconditions.

The pulse may have a temporal distribution curve as shown. While anormal temporal distribution curve is shown, other pulse shapes arepossible. By making the width of the curve 4010 longer (e.g., 3× longer)than the coherence length of pulses that are launched, coherent beaminterference and coherent beam scintillation may be reduced.

FIG. 40 shows an exemplary temporal distribution curve of ashort-duration (e.g., approximately 100 femtosecond) pulse 4010 thattraveled a substantial distance (e.g., one mile) through a variablyrefractive medium and been temporally broadened. The pulse, as itarrives at the photoreceiver, may have a FWHM duration 4030 and acoherence time 4040, which may be equal to a coherence length of thepulse divided by the speed of light through the variably refractivemedium. In some embodiments, the FWHM duration 4030 may be greater thanthe coherence time 4040 of the pulse. Preferably, the FWHM duration 4030may be at least 2×, at least 3×, at least 4×, at least at least 6×, atleast 8×, at least 10×, or at least 12× the coherence time 4040 of thepulse. By ensuring that the FWHM duration 4030 of the pulse as receivedat the photoreceiver is relatively large as compared to the coherencetime 4040 of the pulse 4010, interference between the different raypaths of the pulse as they arrive at the photoreceivers at differenttimes may be reduced, and a signal with reduced noise and higher qualitymay arrive at the photoreceiver.

The photoreceiver may have a detection window 4020 of a specifiedduration. A shorter detection window generally allows higher datathroughput. For example, in a system that uses on-off keying for datamodulation, a photoreceiver having a detection window of 1 nanosecondcan extract up to 1 Gbps while a photoreceiver having a detection windowof 100 picoseconds can extract up to 10 Gbps. The photoreceiver may haverepeating detection windows of less than 100 ns, less than 10 ns, lessthan 1 ns, less than 100 ps, or less than 10 ps.

Pulse length and temporal broadening can, however, cause photons from apulse intended to be received in one detection window to fall into anadjacent detection window. In the case where the adjacent detectionwindow should not receive transmitted photons (e.g., because a ‘0’ istransmitted in that bit position), this phenomenon can produce biterrors. Accordingly, to maximize data transmission accuracy, it isimportant that the FWHM duration 4030 of the pulse as received at thephotoreceiver be greater (and preferably at least three times as large)than the coherence length 4040 of the pulse, while at the same time, theFWHM duration 4030 of the pulse as received at the photoreceiver shouldalso be substantially less than the detection window 4020 of thephotoreceiver.

For example, the detection window 4020 may be at least 2×, at least 5×,at least 6×, at least 7×, at least 8×, at least 10×, or at least 20× aslarge as the FWHM duration 4030 of the pulse as received at thephotoreceiver. Preferably, at least 95%, at least 99%, or at least99.99% of the photons in a pulse that arrive at the photoreceiver mayarrive at a respective arrival time that is spaced from a center 4040 ofthe temporal distribution curve of the pulse by a respective timedifference that is less than half of the detection window duration ofthe photoreceiver. Note that although the center 4040 of the temporaldistribution curve of the pulse is shown at the center of the detectionwindow 4020, this need not be the case, and pulses may arrive earlier orlater than the midpoint of a detection window. It may be preferable thatthe center 4040 of the temporal distribution curve be at or near thecenter of the detection window 4020 to reduce the potential for photonsin a pulse to spill over into an adjacent detection window. In someembodiments, the center 4040 of the temporal distribution curve may beless than 100 picoseconds, 50 picoseconds, 20 picoseconds, 10picoseconds, 5 picoseconds, 1 picosecond, 800 femtoseconds, or 500femtoseconds from the center of the detection window 4020.

By specifying relationships between the coherence time 4040 of thepulse, the FWHM duration 4030 of the pulse as it arrives at thephotoreceiver, and the detection window 4020 of the photoreceiver in themanner described herein, data transmission accuracy and effectivetransmission range can be greatly improved (see below discussion withrespect to FIG. 42 for test results). The FWHM duration 4030 of thepulse as it arrives at the photoreceiver may vary depending on the pulselength as transmitted from the source, the medium through which thepulse travels (e.g., atmospheric pressure, temperature, sunlightintensity, aerosols), and the distance over which the pulse travels toreach the photoreceiver. Accordingly, the coherence time 4040 of thepulse may need to be decreased and/or the detection window 4020 of oneor more photoreceivers may need to be increased depending on conditionsfor the optical communication system. Decreasing coherence time 4040 andincreasing detection window 4020 may thus improve data transmissionquality while negatively impacting data throughput. In some embodiments,the system may be configured to determine a data transmission quality ofthe system (e.g., a bit error rate or a measurement of signal valuesabove or below a detection threshold), and in response to the determineddata transmission quality, modify either or both of the coherence time4040 of the pulse or the detection window duration 4020 of thephotoreceiver.

Similarly, when using a source that can continuously emit light, such asa continuous wave source or a superluminescent diode, the emitted lightcan be gated into pulses (or otherwise converted into pulses using datamodulation or other known techniques) that occupy only a relativelysmall fraction of the duration of the detection window, and those pulsesmay be timed to arrive at or near the centers of the detection windowsof the photoreceiver. Gating and timing the pulses in this manner canreduce the risk that photons in an “on” window (where light is intendedto be transmitted) may spill over into an “off” window (where light isnot intended to be transmitted) and produce bit errors. The pulsedurations and positions relative to the detection windows describedabove may thus also apply to pulses generated using sources that cancontinuously emit light. In such cases, although the sources cancontinuously emit light, the effective output may be “off” for amajority of the time even during “on” transmission windows where lightis intended to be transmitted, so that sufficient space may be leftbetween the center of the pulse and the ends of the detection window toavoid spillover. For example, during an “on” bit window where light isintended to be transmitted, the effective output from the continuousemission source may be “on” less than 75%, 50%, less than 30%, less than20%, or less than 10% of the respective transmission bit window.

FIG. 41 shows a diagrammatic representation of light pulses arriving indetection windows 4020 a, 4020 b, 4020 c of a photoreceiver. The lightpulses may be of any shape and generally may be broadened to some extentby traveling over an optical communication distance through a variablyrefractive medium. In a first detection window 4020 a, a light pulse mayarrive at or near the center of the window and may cause the totalreceived light in that window to exceed a detection threshold Vth, whichmay be processed by circuitry of the photoreceiver to indicate that apulse was received in that window. In some embodiments, this may causethe photoreceiver to output a ‘1’ for this detection window. At the endof detection window 4020 a and before detection window 4020 b, thephotoreceiver circuit may be reset and return to zero. In detectionwindow 4020 b, no pulse is transmitted (e.g., because a ‘0’ is intendedto be transmitted and a modulator at the transmit node blocked thepulse), and the total light received in window 4020 b may be below thedetection threshold Vth. This may cause the photoreceiver to output a‘0’ for this detection window. The photoreceiver circuit may again bereset and return to zero, and the cycle may repeat with a third window4020 c, and so on.

The detection threshold Vth may be configured so that it is sufficientlyhigh that environmental light will not trigger a false positive butsufficiently low that true pulses will reliably exceed the detectionthreshold Vth. It is important that pulses sufficiently exceed a noisefloor so that there is sufficient signal difference between “on” and“off” bit windows so that the detection threshold V t h may be both highenough to ignore environmental noise but low enough to capture everytransmitted pulse. This is particularly challenging over longerdistances (e.g., a mile or more) and in suboptimal environmentalconditions (e.g., partly sunny, significant aerosols). The relationshipsbetween pulse length at the photoreceiver, coherence time, and detectionwindow described herein with respect to FIGS. 39-41 greatly improvesignal quality transmission and allow effective detection thresholds Vtheven for free space optical systems transmitting data over opticaltransmission distances in excess of 1 mile, 2 miles, 3 miles, 5 miles,or 7 miles.

In a case with a beam splitter and multiple photoreceivers, each of themultiple photoreceivers may generate a bit stream based on the separatedpulses that are directed to that photoreceiver, and the bit streams fromthe respective photoreceivers may be interleaved to produce a combinedbit stream having a higher data rate. The combined bit stream may beoutputted to a communication network as described above, including withrespect to FIG. 36 .

FIG. 42 shows an example of test data received over a one-mile opticalcommunication distance. The test data compares optical signals generatedusing a transmit node as described above with respect to FIG. 35Aagainst optical signals generated using a continuous wave source havingthe same average power as the USPL source. Specifically, to generate thedata shown in the top row of the chart shown in FIG. 42 , a USPL sourceincorporated in a transmit node as described above with respect to FIG.35A was used to transmit data over an optical communication distance ofone mile. The received signal was directed at a piece of white paper,and an infrared camera was placed behind the paper to record the lightthat passed through the paper. To generate the data shown in the bottomrow of the chart shown in FIG. 42 , the same experimental setup was usedwith a continuous wave source having the same average power and sameoptical communication distance as the USPL source. The light from boththe USPL source and the CW source was directed at the same sheet ofwhite paper, and the two signal spots were captured in the same frameusing the infrared camera. The spot sizes were approximately 12 inchesin diameter. Background environmental light was subtracted from eachpixel, and a each pixel was subjected to a thresholding logic such thatpixels in which the received optical signal was above the threshold wereset to “white” and pixels in which the received optical signal was belowthe threshold were set to “black.” The four images shown for each sourcewere taken from the same frames in the video feed, and those frames wereequally spaced at intervals of 10 seconds. Frame A shows the receivedsignals from the USPL and CW sources at 10 seconds, Frame B shows theshows the received signals from the USPL and CW sources at 20 seconds,Frame C shows the shows the received signals from the USPL and CWsources at 30 seconds, and Frame D shows the shows the received signalsfrom the USPL and CW sources at 40 seconds.

This data shows that the transmit node as described herein producesultrashort pulses that are substantially more clustered and, within thedetection field, much more reliably exceed the detection threshold. Asapplied to a communication system using a photoreceiver having thecharacteristics described above, including with reference to FIGS. 35Bto 41 , this produces vastly improved data transmission accuracy.Applicant's testing of systems in accordance with this description hasdemonstrated free space optical communication distances in excess of 1mile, 2 miles, 3 miles, 5 miles, and up to as much as 7.4 miles withzero bit error rate as measured over time intervals of at least 10seconds, at least 30 seconds, at least 60 seconds, at least 10 minutes,at least 30 minutes, and at least 1 hour. In some embodiments, systemsdescribed herein may transmit data over an optical communicationdistance of at least one mile and have a measured bit error rate of lessthan one in one million, less than one in one billion, less than one inone trillion, or less than one in one quadrillion over a measurementperiod of at least sixty seconds. To Applicant's knowledge, no otherfree space optical system has achieved similarly low over opticalcommunication distances of even one half of one mile.

Thus, the systems described herein allow for substantially improved datatransmission accuracy, communication link distance, and they also allowfree space optical communication to be used in inclement environmentalconditions (e.g., rain, fog, atmospheric scintillation) that, in priorsystems, rendered free space optical communication ineffective. In someembodiments, the improved data transmission quality and range may alsoallow for free space optical communication to be applied to systems thatwould have previously been impossible to use effectively. For example, atransmit node and/or receiving node in accordance with the presentdisclosure may be provided in an Earth-orbiting satellite to provide forground-to-space and/or space-to-ground free space optical communication.Due to the amount of atmosphere that a beam must travel between Earth'sground level and space, effective optical data transmission has not beendemonstrated using technologies prior to the present disclosure, but thetechnology described herein can achieve effective optical communicationover this distance.

FIG. 43 shows an exemplary ranging node 4400 that can be used to detectobjects or surfaces and determine positions of those objects relative tothe node. The ranging node 4400 may generally include the components ofthe transmit and receiving nodes 3510, 3530 described above with respectto FIGS. 35A and 35B. For example, the ranging node 4400 may include asource 3512, a splitter, one or more modulators, an amplifier, and atelescope. These elements may collectively be configured to emit opticalpulses that travel through a variably refractive medium toward a surfaceS. In the case of a laser ranging node, data modulation is optional butmay be included to encode information relating to the pulses, nodes, orother information. Photons from the optical pulses may be reflected bysurface S and return to the node 4400. The total travel distance of theoptical pulses from transmission by the ranging node to receipt of thereflected pulse may be twice the distance of the node to the surface S.Upon return to the node, the pulses may be received by an aperture 3532,optionally split by a splitter 3534, and analyzed using one or morephotoreceivers 3536. Each of these components may have the sameproperties and parameters as the corresponding components describedabove with respect to FIGS. 35A to 41 . Ranging node 4400 mayadditionally include a time-of-flight (TOF) circuit 4410, which may beconfigured to determine the time of flight of a pulse to reach surface Sand return to node 4400, and thereby determine a distance of thatsurface S from the ranging node 4400.

One or more aspects or features of the subject matter described hereincan be realized in digital electronic circuitry, integrated circuitry,specially designed application specific integrated circuits (ASICs),field programmable gate arrays (FPGAs) computer hardware, firmware,software, and/or combinations thereof. These various aspects or featurescan include implementation in one or more computer programs that areexecutable and/or interpretable on a programmable system including atleast one programmable processor, which can be special or generalpurpose, coupled to receive data and instructions from, and to transmitdata and instructions to, a storage system, at least one input device,and at least one output device.

FIG. 44 shows an exemplary optical communication system for transmittingdata through a medium. As shown in FIG. 44 , the system may include oneor more of the following: an optical source 4412, a modulator 4414, anamplifier 4416, one or more telescopes 4417, 4419, a testing module4422, a network data input 4418, an error detector 4420, a photoreceiver4426, a power gauge 4424, and a display 4428.

The optical source 4412 may include a structure capable of generating abeam of light with a short coherence length. Preferably, the coherencelength of the emitted beam may be less than 1 mm, less than 600 microns,less than 400 microns, less than 200 microns, less than 100 microns,less than 50 microns, or less than 10 microns. In some embodiments, thebeam, when emitted through a variably refractive medium over atransmission distance, may have the parameters and properties describedabove with respect to FIGS. 38-42 .

In some embodiments, the source 4412 may include a photon-emittingsurface and a waveguide for amplifying the emitted light, as describedbelow with respect to FIG. 45 . In some embodiments, the source may beor include one or more superluminscent diodes (SLEDs). In otherembodiments, the source 4412 may include a laser diode configured topump light into an amplifier fiber doped with a transition metal ioncompound, such as erbium, as described below with respect to FIG. 46 .

The source 4412 may have an optical bandwidth at least as large as anoptical gain bandwidth of amplifier 4416. By matching the opticalbandwidth of source 4412 to the optical gain bandwidth of amplifier4416, energy applied to source 4412 and amplifier 4416 may be moreefficiently used to generate a beam of light, and the resulting lightmay have a minimal coherence length. The source may be capable ofcreating high peak power pulses, such as at least 5 W, at least 20 W, atleast 50 W, at least 100 W, at least 500 W, at least 1 kW, at least 5kW, at least 10 kW, at least 50 kW, at least 100 kW, at least 500 kW, orat least 1 MW.

In some embodiments, source 4412 may generate substantial noise in theform of randomly fluctuating power in the beam of light. The noise maybe high-frequency noise such as “white” noise or “pink” noise.

The source 4412 may emit a beam of light. The beam of light may be sentto a modulator 4414. The modulator 4414 may encode transmission datainto a plurality of time slots associated with the beam. In someembodiments, the modulator may be a Mach-Zehnder Modulator (MZM). Insome embodiments, the modulator 4414 may be configured to encode datausing on-off keying. For example, the transmission data may be a bit (a0 or 1). The modulator 4414 may determine that at a first-time intervalassociated with the beam, the bit should be encoded 1, which mayindicate “on.” The modulator 4414 may determine that at a second-timeinterval associated with the beam, the bit should be encoded 0, whichmay indicate “off.” The modulator 4414 may encode the bit by eitherblocking or allowing the beam of light to pass through the modulator4414 at a given time slot. For example, if the bit encoded should be 0,the modulator 4414 may block the beam of light from passing through. Ifthe bit encoded should be 1, the modulator 4414 may allow the beam oflight to pass through.

In some embodiments, the data to be encoded in the beam of light may bereceived from a network data input 4418. Network data may be encoded inthe beam of light by modulator 4414 or may be encoded by directlymodulating source 4412. In cases where source 4412 is directlymodulated, modulator 4414 may be omitted.

As described above, light emitted by source 4412 may be gated intopulses (or otherwise converted into pulses using data modulation orother known techniques) that occupy only a relatively small fraction ofthe duration of a detection window of a photoreceiver 4426. In someembodiments, those pulses may be timed to arrive at or near the centersof the detection windows of the photoreceiver. In such cases, althoughsource 4412 may continuously emit light, the effective output may be“off” for a majority of the time even during “on” transmission windowswhere light is intended to be transmitted. For example, during an “on”bit window where light is intended to be transmitted, the effectiveoutput from the continuous emission source may be “on” less than 75%,50%, less than 30%, less than 20%, or less than 10% of the respectivetransmission bit window.

The source 4412 may thus be time-sliced. In some embodiments, this maybe accomplished using a Mach-Zehnder interferometer (MZI) or anelectroabsorption modulator. In some embodiments, the source 4412 may bedirectly modulated to achieve a time-sliced behavior. For example, anMZI or electroabsorption modulator could be used in conjunction with anelectrical “comb” generate that is synchronized to the data modulationcircuit to provide the pulse slicing.

The output of the modulator 4414 may be a modulated beam of light. Themodulated beam of light may have a relatively low signal-to-noise ratio(SNR), as described below with respect to FIGS. 49A-49F. For example,the modulated beam of light output by the modulator 4414 may have a SNRless than 5, less than 4, less than 3, less than 2, less than 1, or lessthan 0.5.

In some embodiments, a modulator 4414 may be omitted from the opticalcommunication system. In some embodiments, data may alternatively oradditionally be encoded into the beam of light by directly modulatingthe source 4412. For example, a bit stream may be converted to a seriesof instructions to turn off or turn on the source 4412 in respectivetimeslots, such that data is encoded into the beam of light using on-offkeying without a modulator 4414.

The modulated beam of light may be sent to the amplifier 4416. In someembodiments, amplifier 4416 may be a fiber amplifier as described belowwith respect to FIG. 47 . Amplifier 4416 may amplify and filter themodulated beam signal. Due to the filtering by amplifier 4416, theamplified signal output from amplifier 4416 may have a substantiallyimproved signal to noise ratio as compared to the input, as described ingreater detail below with respect to FIGS. 49A-F. The fiber amplifier4416 may thus amplify the beam of light (e.g., ASE signal) whilereducing high-frequency noise associated with the beam of light.

The filtered beam of light may be transmitted from the fiber amplifier4416 to a detector which may include a photoreceiver 4426 and a powergauge 4424. The filtered beam of light may be transmitted through amedium with a variable (e.g., randomly variable) refractive index. Thephotoreceiver 4426 may extract the transmission data from the filteredbeam of light. The power gauge 4424 may include an optical power meterto measure the power in the received signal. The beam of light receivedby the photoreceiver 4426 may have one or more characteristics describedwith respect to FIGS. 39 through 41 .

The system may optionally include one or more of a testing module 4422,a network data input 4418, and an error detector 4420. The testingmodule 4422 may be configured to measure a bit error rate of thecommunication system. Testing module 4422 may also be used to test otherparameter of the communication system or the signal received attelescope 4419. Network data input 4418 may generate a test pattern tobe used by the system. Error detector 4420 may determine a number of biterrors produced by the communication system. A clock signal generatormay be used to synchronize the network data input 4418 and the errordetector 4420. The display 4428 may output the received signal and mayinclude analysis of the signal as related to the network data input4418. The testing module 4422 may include electrical-optical convertersconfigured to test optical communication signals.

FIG. 45 shows an exemplary optical source configured to generate anamplified spontaneous emission (ASE) output. The optical source 4500 maycontain semiconductor layers such that, when a current is applied, asurface of the source emits photons. The emitting surface may bedisposed along an interior of a waveguide 4502, such that emittedphotons travel within the waveguide. In some embodiments, the waveguidecore may be fluorescent. The waveguide 4502 may confine the fluorescenceproduced as current is applied within the body of the structure. Thewaveguide 4502 may include an open end and a closed end. As current isapplied, at least a portion of the fluorescence may propagate towardsthe closed end and may be reflected by a reflector 4504. A beam of light4508 may be emitted from the open end of the waveguide 4502. The use ofwaveguide 4502 and reflector 4504 may amplify the emitted light with again proportional to the length of the waveguide 4502.

In some embodiments, the beam of light 4508 may be in the visible orinfrared spectrums. In some embodiments, other spectra may be used. Insome embodiments, the beam of light may have a relatively shortcoherence length and/or broad spectral bandwidth, as described above,including with respect to FIGS. 35-43 . In some embodiments, the light4508 may include a FWHM spectral bandwidth of at least 10 nanometers, atleast 20 nanometers, at least 25 nanometers, at least 30 nanometers, atleast 40 nanometers, at least 50 nanometers, at least 70 nanometers, atleast 100 nanometers, or at least 200 nanometers.

FIG. 46 shows another exemplary source configured to be used in acommunication system such as that shown in FIG. 44 . An amplifierincluding doped fiber 4604 (e.g., similar or equivalent to the amplifierdescribed with respect to FIG. 47 ) may be used as a preamplifier togenerate the beam of light. The preamplifier 4500 may be modified toenable it to generate the beam of light with the properties describedwith respect to FIG. 44 . For example, the preamplifier 4500 may be ableto double pass an input laser diode 4602, which may reflect off a backwall reflector 4606 of the preamplifier. An anti-reflection coating maybe applied to the reflector 4606. The preamplifier may be pumped by alaser diode 4602 and may emit an ASE signal that matches the gainspectrum of the amplifier doped fiber 4604.

FIG. 47 shows an exemplary fiber amplifier configured to amplify andfilter a beam of light. As shown in FIG. 47 , the amplifier may includeone or more layers of cladding 4704, one or more cores 4706 configuredto receive and transmit a data-encoded optical signal, pump light 4700,a fiber adjacent to the cladding, and a polymer coating 4708. The fiberamplifier may be a nonlinear filter and may be suited for amplifying aninput signal 4710 while reducing high-frequency noise associated withthe input signal 4710. The nonlinear filter may be an optical devicethat produces an output signal that is not a simple linear mathematicalconstant times the input signal.

The core 4706 may be surrounded by one or more layers of cladding 4704.The core 4706 may be doped with one or more transition metal ions suchas Erbium, Ytterbium, Neodymium, Terbium, and/or the like. In someembodiments, the fiber amplifier may be an erbium doped fiber amplifier.

In some embodiments, the pump light may have a wavelength ofapproximately 980 nanometers. In some embodiments, the pump light mayhave a wavelength of approximately 1480 nanometers. In some embodiments,the pump light may have a wavelength of approximately 980 nanometers.The pump light may excite the transition metal ions in the core. Whenthe excited ions are stimulated by photons in the optical signaltraveling through the core, the ions may emit photons, therebyamplifying the optical signal. In some embodiments, a portion of theions may remain in an excited state for at least a nanosecond, amicrosecond, or a millisecond before emitting photons. In someembodiments, the light emitted by the transition metal ions may havewavelengths that are equal to or within 10 nanometers of the lightemitted by the source. For example, the emitted light may be between1500-1600 nanometers in wavelength.

The beam of light may be amplified and filtered due to its interactionwith the excited ions. The excited ions may cause a gain on the beam oflight, resulting in an increase in power associated with the beam oflight. The gain of the fiber amplifier may multiply the power of theoptical signal by a factor of at least 5, 10, 20, 30, 40, 50, 100, 200,500, 1,000, 5,000, 10,000, or 30,000. The filtering effect of theamplifier is described in greater detail below with respect to FIGS.49A-F.

FIGS. 48A-F show measurements (and representations thereof) of anexemplary continuous wave output. FIGS. 49A-F show measurements (andrepresentations thereof) of an exemplary ASE output.

FIG. 48A shows a measurement of an optical signal emitted by aconventional laser (configured to emit a narrow-band, long coherencelength signal), and FIG. 48D shows an illustration of qualitativelysimilar data. FIG. 49A, by contrast, shows a measurement of an opticalsignal emitted by a high-noise, short coherence length source such asdescribed above with respect to FIGS. 44-46 . FIG. 49D shows anillustration of data that is qualitatively similar to that in FIG. 49A.Comparing FIGS. 48A and 48D to FIGS. 49A and 49D, it can be observedthat the high-noise, short coherence length source can emit an averagepower similar to that of the conventional laser, but with substantiallygreater noise in the form of high-frequency fluctuations in the power ofthe emitted beam.

FIG. 48B shows a measurement of the optical signal produced by theconventional laser after that signal has been modulated using an MZM.FIG. 48B shows several measurement traces, which represent repeatedmeasurements of the modulated signal. FIG. 48E shows an illustration ofa single trace representing data that is qualitatively similar to thatshown in FIG. 48B over a three bit period. In FIG. 48E, bit window 4820a represents an “on” bit, 4820 b represents an “off” bit, and 4820 crepresents an “on” bit. As shown in this diagram, the modulated signalis relatively steady in each window near an “on” power level P_(on) for“on” bits and near an “off” power level P_(off) in “off” bits. Althoughthe conventional laser produces a small amount of noise, this noise issmall as compared to the difference between P_(on) and P_(off),resulting in a relatively high signal-to-noise ratio (SNR).

FIG. 49B shows a measurement of the optical signal produced by thehigh-noise, short coherence length source after that signal has beenmodulated using an MZM. FIG. 48B shows several measurement traces, whichrepresent repeated measurements of the modulated signal. Due to thesignificant noise in the signal, the measurement traces are highlyvariable. FIG. 48E shows an illustration of a single trace representingdata that is qualitatively similar to that shown in FIG. 48B over athree bit period. In FIG. 49E, bit window 4920 a represents an “on” bit,4920 b represents an “off” bit, and 4920 c represents an “on” bit. Asshown in this diagram, the modulated signal centers at or near the “on”power level P_(on) for “on” bits and at or near the “off” power levelP_(off) in “off” bits, but the significant noise in the signal resultsin significant variance above and below the desired power levels. Thissignal noise obscures the difference between the “on” state and the“off” state and, if not corrected, would render the transmission beamunreliable, particularly if transmitted through atmosphere oversignificant distances, which tends to weaken and introduce additionalnoise in the beam before it is received by a photoreceiver. Withreference to FIG. 49E, the SNR of the modulated beam may be defined ashalf of the difference between P_(on) and P_(off) divided by thestandard deviation of the measured power over a given bit window.Applying this definition, the SNR of the modulated beam may be less than5, less than 3, less than 2, less than 1.5, less than 1, less than 0.5,less than 0.2 or less than 0.1.

FIG. 48C shows a measurement of the optical signal produced by theconventional laser after that signal has been modulated using an MZM andthen amplified using a fiber amplifier such as that described above withrespect to FIG. 47 . FIG. 48F shows an illustration of a single tracerepresenting data that is qualitatively similar to that shown in FIG.48C over a three bit period. Comparing FIG. 48B to FIG. 48C and FIG. 48Eto FIG. 48F, it is observed that the SNR remains similar in theconventional laser system both before and after the modulated signal ispassed through the amplifier.

FIG. 49C shows a measurement of the optical signal produced by thehigh-noise, short coherence length source after that signal has beenmodulated using an MZM and then amplified using a fiber amplifier suchas that described above with respect to FIG. 47 . FIG. 49F shows anillustration of a single trace representing data that is qualitativelysimilar to that shown in FIG. 49C over a three bit period. ComparingFIG. 49B to FIG. 49C and FIG. 49E to FIG. 49F, it is observed that theSNR in the system using the high-noise, short coherence length source issubstantially after the optical signal has been passed through the fiberamplifier. The SNR of the amplified, filtered signal may be at least 1,2, 3, 5, 7, 10, 15 or 20. In some embodiments, the SNR of the post-fiberamplifier filtered signal may be at least 2×, 3×, 4×, 5×, 7×, 10×, or20× higher than the SNR of the signal post-modulation butpre-amplification.

FIG. 50 shows an exemplary optical communication system coupled to afiber optic gyroscope (FOG). As shown in FIG. 50 , the opticalcommunication system may include an optical source 5000, a coupler 5002,a photodetector 5004, a modulator 5006, an amplifier 5008, a telescope5010, output 5012 from the telescope, and fiber coils 5014. The FOG maybe analog or digital.

The optical source may be the optical source described with respect toFIGS. 44 through 46 and may include one or more SLEDs to emit a beam oflight.

The beam of light may be sent to a coupler 5002 which may split the beaminto two beams, e.g., a first beam and a second beam. The first beam maytravel clockwise along the coils 5014 while the second beam travelscounterclockwise. Using the phase shift between the two coils 5014, theorientation of the gyroscope may be sensed (e.g., clockwise orcounterclockwise). The FOG may be used on a free space optical system asa tool to calculate the absolute position of anything it is linked to.

The beam of light may be modulated and amplified using techniquesdescribed with respect to FIGS. 44 through 49 . In some embodiments, thefiltered light may be used as input to the coupler 5002 and may be usedby the FOG to sense orientation.

FIG. 51 shows an exemplary flow chart of the optical communicationsystem configured to transmit data.

In some embodiments, a system for transmitting information optically mayinclude an optical source, a modulator, and a photoreceiver.

At step 5100, the optical source may be configured to generate a beam.The beam may include a series of light pulses each having a duration ofless than 100 picoseconds. The optical source may include a waveguidethat amplifies light emitted by one or more diodes. The beam of lightemitted by the optical source may have a coherence length less than 400microns. The beam of light may include high frequency noise in the formof amplitude fluctuations. In some embodiments, the system may include aplurality of optical sources configured to generate respective beams oflight. Each of the plurality of optical sources may include a respectivewaveguide. The respective beams of light may be coupled to a multiportcoupler such that a combined output from one or more of the plurality ofoptical sources is transmitted to the modulator. The combined output maybe used for system redundancy, course wavelength division multiplexing(WDM), hot swapping, and/or the like.

At step 5102, the beam of light may be modulated. A modulator may beconfigured to modulate the series of light pulses in response to a datatransmission signal, thereby encoding transmission data into the seriesof light pulses. The modulator may be configured to encode a bit in agiven time slot by blocking or allowing the beam of light to passthrough the modulator in that time slot. The modulator may output amodulated beam of light having a first SNR.

At step 5104, the modulated beam of light may be received and bothamplified and filtered. A fiber amplifier, which may include at least acore and a cladding surrounding the core, may be configured to receivethe modulated beam of light from the modulator and both amplify andfilter the modulated beam of light to produce a filtered beam of light.The cladding may include a transition metal ion compound. Photons with awavelength between 1500 nm and 1600 nm may be emitted from thetransition metal ion compound and amplify the modulated beam of light bya gain of at least 10×. In some embodiments, the transition metal ionmay be at least one of Erbium, Ytterbium, Neodymium, or Terbium. Thefiltered beam of light may have a second SNR. The second signal-to-noiseratio may be at least three times as large as the first signal-to-noiseratio. In some embodiments, the fiber amplifier may a first fiberamplifier. The system may include a second fiber amplifier. The firstfiber amplifier may transmit the filtered beam of light to the secondfiber amplifier.

At step 5106, the filtered beam of light may be transmitted. Thefiltered beam of light may be transmitted to a detector having aphotoreceiver. The photoreceiver may be configured to extract thetransmission data from the filtered beam of light. In some embodiments,the optical source and the detector having the photoreceiver may bespaced by a free space optical communication distance of at least onemile. The optical communication system may have a measured bit errorrate of less than one in one million over the free space opticalcommunication distance of at least one mile for a measurement period ofat least sixty seconds. In some embodiments, the optical source may belocated on a ground station and the photoreceiver is disposed on anearth-orbiting satellite. The optical communication system may have ameasured bit error rate of less than one in one billion over a freespace optical communication distance between the ground station and theearth-orbiting satellite for a measurement period of at least sixtyseconds. The optical source may a SLED and the fiber amplifier may anonlinear filter that amplifies the modulated beam of light and reducesthe high frequency noise.

The optical source may be configured to generate a beam comprising aseries of light pulses each having a duration of less than 100picoseconds. The modulator may be configured to modulate the series oflight pulses in response to a data transmission signal, thereby encodingtransmission data into the series of light pulses. The photoreceiver mayhave a detection window duration of less than 1 nanosecond and adetection threshold. The photoreceiver may be configured to indicatewhether a received optical energy during a given detection window isgreater than the detection threshold. The series of light pulses mayinclude a first light pulse having a coherence length of less than 400microns. When the first pulse travels through the variably refractivemedium, photons in the first pulse may be refracted to travel alongdifferent ray paths having different lengths to the photoreceiver, andthe photons of the first pulse may arrive at the photoreceiver accordingto a temporal distribution curve that depends, at least in part, on theduration of the first pulse and the lengths of the different ray pathstaken by the photons in the first pulse to the photoreceiver. A fullwidth at half maximum (FWHM) value of the temporal distribution curvemay be at least three times as large as a coherence time value equal tothe coherence length of the first pulse divided by the speed of lightthrough the variably refractive medium, and the detection window of thephotoreceiver may be at least six times as large as the FWHM value ofthe temporal distribution curve.

In some embodiments, a laser ranging system may include an opticalsource and a photoreceiver. The optical source may be configured togenerate a beam comprising a series of light pulses each having aduration of less than 100 picoseconds. The photoreceiver may have adetection window duration of less than 1 nanosecond and a detectionthreshold. The photoreceiver may be configured to indicate whether areceived optical energy during a given detection window is greater thanthe detection threshold. The series of light pulses may include a firstlight pulse having a coherence length of less than 400 microns. When thefirst pulse travels through the variably refractive medium, photons inthe first pulse may be refracted to travel along different ray pathshaving different lengths to the photoreceiver. The photons of the firstpulse may arrive at the photoreceiver according to a temporaldistribution curve that depends, at least in part, on the duration ofthe first pulse and the lengths of the different ray paths taken by thephotons in the first pulse to the photoreceiver. A full width at halfmaximum (FWHM) value of the temporal distribution curve may at leastthree times as large as a coherence time value equal to the coherencelength of the first pulse divided by the speed of light through thevariably refractive medium, and the detection window of thephotoreceiver may be at least six times as large as the FWHM value ofthe temporal distribution curve. The laser ranging system may beconfigured to transmit the series of light pulses toward a surface,receive at least a portion of the series of light pulses that have beenreflected by the surface, and, based on a time of flight of the receivedportion of the series of light pulses, determine a distance of at leasta portion of the surface from the laser ranging system.

Spectrally-Equalizing Amplifier

FIGS. 52 and 53 depict features for optically transmitting data througha variably refractive medium using a spectrally-equalizing amplifier.The features of FIGS. 52 and 53 may apply or use any feature of FIGS.35A-51, 54-57, and 58A-59 .

FIG. 52 shows an exemplary system incorporating a spectrally-equalizingamplifier configured to equalize a gain of a beam of light andrespective distribution curves of wavelengths. The system may include asource 5200, a modulator 5202, an amplifier 5204 which may be configuredto perform spectrally-equalizing as shown as step 5206, and an output.The output may produce a beam of light with an output spectrum 5208. Theoutput spectrum 5208 may follow a spectrally-equalizing spectrum withrespect to wavelength and intensity. The output spectrum 5208 may bedifferent than a non-spectrally-equalizing amplifier spectrum (e.g., ofan EDFA). The spectrally-equalizing amplifier may provide certainsignificant benefits for the optical communication systems, such as alarger range of wavelengths to transmit data over. As an example, again-flattened spectrum graph 5212 may be compared to anon-spectrally-equalizing amplifier graph 5210 to demonstrate thedifferent properties of the spectrums. Furthermore, the broadband lighthas a short optical coherence length, which is advantageous forfree-space optical communications. The coherence length of the source isdetermined as:

${L = {C\frac{\lambda^{2}}{\Delta\lambda}}},$where C is a shaping constant equal to ½, λ is the central wavelength ofthe pulse, and Δλ is the full width at half maximum (FWHM) spectralwidth of the pulse. In some embodiments, the coherence length may beless than 1 mm, less than 600 microns, less than 400 microns, less than200 microns, less than 100 microns, less than 50 microns, or less than10 microns. In embodiments where a continuous wave source is used, thesevalues may refer to the coherence length of the continuous wave beam,rather than that of the pulses.

A source 5200 may be an optical source configured to generate a beam oflight. The source 5200 may include a waveguide that amplifies emittedlight. In some embodiments, the source 5200 may be a SLED, fiber ASEsource (described above), a mode-locked laser, and/or the like. In someembodiments, the source 5200 may be a SLED described with respect toFIGS. 42 and 45 . In some embodiments, the source 5200 may be a USPL andmay generate ultra-short pulses described with respect to FIGS. 40-42 .In some embodiments, the beam of light emitted by the optical source mayhave a coherence length less than a threshold, for example, 400 microns.In some embodiments, the beam of light may be associated with highfrequency noise in the form of amplitude fluctuations.

A modulator 5202 may be configured to modulate the beam of light toencode data on the beam of light. The data may include transmitted datawhich may include encoding data and/or tracking data. The optical systemmay be operated as a LIDAR instrument, using USPL laser sourcesoperating over a spectral range of interest. In some embodiments, themodulator 5202 may encode the transmitted data into a plurality of timeslots. For example, encoding a bit in a given time slot may involveblocking or allowing the beam of light to pass through the modulator5202 in that time slot. The modulator 5202 may be the modulator 4414described with respect to FIG. 44 . Similarly to what was described withrespect to FIGS. 44 , in some embodiments, the modulator 5202 may not beincluded in the system.

The amplifier 5204 may receive the beam of light from the source 5200or, in some embodiments, the modulator 5202. In some embodiments, theamplifier 5204 may be and have the components of the amplifier 5204described with respect to FIGS. 35A, 35B, 44 and 47 . For example, theamplifier 5204 may be an EDFA. In some embodiments, the amplifier 5204may be a Raman amplifier.

In the case of the Raman amplifier, gain-equalizing may be performed byimplementing and coupling multiple pump wavelengths during theamplification process. For instance, by coupling multiple pump sources(with same or different wavelengths), and pumping light from the pumpsources into the Raman amplifier (along with the signal from themodulator 5202), the Raman amplifier may gain-equalize the spectrum ofthe signal (e.g., the encoded beam of light or plurality of pulses). Forinstance, using FIG. 47 as an example, the pump light 4700 may includetwo or more pump sources to thereby provide same or different spectrumsof pump energy using same or different wavelengths of light.

In some embodiments, the amplifier 5204 may be considered aspectrally-equalizing optical gain amplifier and may equalize the gain.The gain may be equalized to account for an atmospheric condition, forexample, a sunlight, humidity, and the like. In such a case, the gainequalization performed by the amplifier 5204 may equalize the gainacross a spectral domain without completely flattening it. In someembodiments, the gain may be equalized by flattening it across aspectral domain.

In some cases, the amplifier 5204 may include additional componentsand/or features in order to equalize the gain of the received beam oflight/plurality of pulses. Examples of the components and/or featuresmay include, but are not limited to, one or combinations of: long-periodfiber, thin-film interference coating, one or more MEMs arrays forcreating continuous spectrally-dependent reflectivity, acousto-opticequalizer, multiple filtering, and/or the like.

In the case of long-period fiber grating, the long-period fiber gratingmay apply one or more techniques related to graphic equalizing such asrejecting light near the peak of the spectrum while allowing otherwavelengths to pass through. For instance, the amplifier 5204 mayinclude long-period grating in a core of the amplifier 5204, or aseparate long-period fiber grating may be connected before or after theamplifier 5204 (as the case may be, e.g., if the amplifier 5204 hasmultiple stages).

In the case of thin-film interference coating, the thin-filminterference coating may include at least two layers (e.g., a pluralityof layers) of different materials with different indexes of refraction.Based on the materials and sequence of layers, the thin-filminterference coating may (in transmission or reflection) filter thespectrum of the amplifier 5204. The thin-film interference coating maybe integrated into the amplifier 5204 (e.g., in between stages), orapplied to an input and/or out end of the amplifier 5204.

In the case of one or more MEMs arrays for mirroring, the MEMs arraysmay include optical filtering structures (optionally withelectro-mechanical adjustments) and/or semiconductor filteringstructures (e.g., with voltages applied to electrodes on a semiconductorsubstrate stack) to filter the spectrum, of the spectrum of theamplifier 5204. The MEMs arrays may be fixed or dynamically controlled(e.g., to adjust a filter of the MEMs array). The MEMs arrays may beintegrated into the amplifier 5204 (e.g., in between stages), or appliedto an input and/or out end of the amplifier 5204.

In the case of an acousto-optic equalizer, the acousto-optic equalizermay include at least one piezoelectric transducer attached tomaterial(s) (e.g., glass), and (optionally) an acoustic absorber. Theacousto-optic equalizer may control the at least one piezoelectrictransducer to vibrate to cause sound waves in the material(s), therebycausing a filter of spectrum passing through the material(s). Theacousto-optic equalizer may be static (e.g., constant vibration) ordynamically controlled (e.g., to adjust a filter of the acousto-opticequalizer). The acousto-optic equalizer may be integrated into theamplifier 5204 (e.g., in between stages), or applied to an input and/orout end of the amplifier 5204.

In the case of multiple filtering, the multiple filtering may applysuccessive and/or parallel stages of the above techniques to amplify andfilter the spectrum to obtain the output spectrum 5208.

In some embodiments, the gain-flattening components and features may bepositioned and performed in before, after, or in between different gainstages of the amplifier 5204. In some cases, the wavelength of the beamof light emitted by the source 5200 matches and/or corresponds to thewavelength applied by the amplifier 5204. In some embodiments,gain-flattening may be an external process that is independent of theamplifier 5204 and that may occur before or after the beam of the lightis processed by the amplifier 5204. In some embodiments, an optical gainof at least one of 10, 100, 1000, 2000, 5000, and/or the like may beapplied to the received beam of light. The optical gain may be used toincrease its power from a mW range to a Watt range (or more). In someembodiments, the amplifier 5204 may include a linear and/or a nonlinearfilter. For example, a non-linear filter described with respect to FIG.44 . The non-linear filter may amplify the beam of light and reduce highfrequency and/or low intensity noise while saturating the high intensitynoise based on using techniques described with respect to FIGS. 48A-Fand 49A-F. In some embodiments, pump power may be coupled to the filterand controlled and/or adjusted to tune the filtering, for example, thesaturation level applied by the filter.

At 5206, spectrally-equalizing techniques may be applied to the beam oflight, which may result in an outputted beam of light with an increased,or in other words larger, bandwidth. An example of spectral-equalizationmay be gain-flattening. The gain-flattening may occur over the spectraldomain of the beam of light. Gain-flattening the beam of light mayinvolve making the bandwidth of the amplifier 5204 larger. Techniquesmay include equalizing the gain applied by the amplifier 5204 to thebeam of light, thereby making the bandwidth of the amplifier 5204 flatacross the whole (or a substantial portion of the) bandwidth. Becausethe full bandwidth of the amplifier 5204 may be available, a beam oflight may be received by the source 5200 (e.g., the SLED described withrespect to FIGS. 43 and 44 ), or in some embodiments the modulator 5202,and amplified such that the beam of light maintains the shape it hadwhen outputted by the source. In some embodiments, a distribution curveof wavelength of EM radiation may be modified (e.g., flattened) across adefined range. For example, the defined range may be a range above apower threshold. In this manner, a usable range of an optical source andamplifier for data transmission may be increased from 10 or 15 nm to 50nm of more, thereby increasing data bandwidth over the opticalcommunication distance.

An output with a corresponding output spectrum 5208 may be outputted andtransmitted to a detector. Transmission data may be extracted from thebeam of light, for example, via a photoreceiver coupled to the detector,which may be described with respect to FIG. 44 . In some embodiments,the source 5200 and the detector having the photoreceiver are spaced bya distance of one or more of the following ranges: 1 cm to 1 m, 1 m to100 m, 100 m to 500 m, 500 m to 1 km, at least 1 km, and the like. Thesystem may have a measured bit error rate of less than one in onemillion, one in one billion, one in one trillion, and the like for ameasurement period of at least sixty seconds, for a measurement periodof minutes, or for a measurement period of hours.

As shown by comparing non-spectrally-equalizing amplifier graph 5210with the spectrally-equalizing amplifier graph 5212, increasing thebandwidth may in turn shorten the coherence length of the beam of light.In turn, shortening the coherence length of the beam of light mayenhance one or more properties related to signal quality of the beam oflight such as lowering BER, improving the link quality, increasing thesignal's resistance to atmospheric distortions and/or the like. In someembodiments, the coherence length may be shortened to a range ofapproximately 10-15 microns. The benefits of shortening the coherencelength may be described further with respect to FIGS. 37-41 and 42-49 .In some cases, the non-spectrally-equalizing amplifier graph 5210depicts the spectrum of a beam of light that has been amplified using anamplifier 5204 without gain-flattening capabilities, while thegain-flattened spectrum graph 5212 depicts the spectrum of a beam oflight amplified by a gain-flattened amplifier. With respect to thenon-spectrally-equalizing amplifier graph 5210, a spike (e.g., a peak ingain) in intensity may occur on the spectrum at a wavelength of around1531 nm and then the intensity may drop at higher wavelengths (e.g., ashoulder of gain), before the intensity trails off to the right. Withrespect to the gain-flattened spectrum graph 5212, intensity ismaintained at the higher wavelengths. In some cases, the maintainedintensity may lead to the beam of light having a shorter coherencelength. In some embodiments, the gain may be flattened such that theoutput power density is within a plus or minus 0.5 dB, 1 dB, 2 dB, 3 dB,4 dB, etc. across a specified spectral range, for example, a 20, 30, 40,50 nm bandwidth, or more. In other words, the wavelengths of thefiltered beam of light may be made to have equal spectral power density,which may be understood as power per channel or nm band (with at least adefine threshold amount of power per channel).

In some cases, based on spectrally equalizing the gain applied toencoded beam of light/encoded beam of light, amplified outputwavelengths within a defined range (e.g., ±50 nm of a center wavelength)are made to have substantively equal spectral power density. In somecases, the gain applied to the encoded beam of light is flattened toproduce a distribution curve of wavelengths associated with the filteredbeam. For instance, the distribution curve may have a variance less than±0.05 dB, 1 dB, or 2 dB, and the like, over the defined range ofwavelengths.

As mentioned above, the optical source may be a superluminescent diode(SLED). In some cases, the spectrally-equalizing amplifier is anonlinear filter that amplifies the encoded beam of light and reduceshigh frequency noise (e.g., a EDFA with additional features) or a Ramanamplifier.

In some cases, the filtered beam of light comprises a plurality ofpulses including at least a first pulse that is transmitted over anoptical communication distance to the photoreceiver. As the first pulsetraverses over the optical communication distance, photons, of the firstpulse, may travel along a plurality of ray paths having differentlengths to the photoreceiver. The photons of the first pulse of lightmay arrive at the photoreceiver according to a temporal distributioncurve that depends, at least in part, on a duration of the first pulseand the different lengths of the plurality of ray paths taken by thephotons in the first pulse to the photoreceiver. In this case, a fullwidth at half maximum (FWHM) value (see, e.g., FIGS. 35A-51 ) of thetemporal distribution curve is at least three times as large as acoherence time value equal to a coherence length of the first pulsedivided by a speed of light through the variably refractive medium.

FIG. 53 shows an exemplary method of spectrally equalizing a gain of abeam of light using a spectrally-equalizing amplifier. The method maystart at step 5300, the system may, by an optical source, generate abeam of light. For instance, the optical source may be a SLED, asdiscussed herein.

In step 5302, the system may encode, by a modulator, data on the beam oflight to produce an encoded beam of light. For instance, the modulatormay receive a data signal and on-off key (e.g., return to zero) the beamof light in accordance with the digital signal and time slots, asdiscussed herein.

In step 5304, the system may amplify and filter, by aspectrally-equalizing amplifier, the encoded beam of light to produce afiltered beam of light. The spectrally-equalizing amplifier mayspectrally equalize a gain applied to the encoded beam of light. Forinstance, the spectrally-equalizing amplifier may apply a gain to thebeam of light that ensures the output spectrum has at no more than ±1 dBof variance over a target range of wavelengths, as discussed herein.

In step 5306, the system may transmit the filtered beam of light througha variably refractive medium to a detector having a photoreceiver. Forinstance, the system may route the filtered beam of light to a telescopethat transmits the filtered beam of light, e.g., to a detector, asdiscussed herein.

Temperature Controller

FIGS. 54, 55, 56A, 56B, and 57 depict features for opticallytransmitting data through a variably refractive medium using atemperature controller of an optical source. The features of FIGS. 54,55, 56A, 56B, and 57 may apply or use any feature of FIGS. 52-53, and58A-59 .

FIG. 54 shows an exemplary system for tuning a temperature of an opticalsource to optically transmit data through a refractive medium. As shown,the system may include an optical source 5400, a temperature controller5402, a heater/cooler 5404, and a thermometer 5406. In some embodiments,the system may include additional features such as an RF connector (SMAconnector), a DC drive bias, and/or the like.

An optical source 5400 may be configured to generate a beam of light.The beam of light may have a spectrum of wavelengths. In someembodiments, the optical source 5400 may be a semiconductor. In someembodiments, the optical source 5400 may be a SLED, for example, theSLED described with respect to FIGS. 44 and 45 . As described withrespect to FIG. 55 , multiple sources, for example, multiple SLEDs, maybe used in combination. In some embodiments, the optical source 5400 maybe a fiber ASE source (described above), a mode-locked laser, and/or thelike.

A temperature controller 5402 may be coupled to the optical source 5400in such a way that it can adjust the temperature of the optical source5400 and, in doing so, drive the beam it generates to have a certainrange of wavelengths. In some embodiments, the temperature controller5402 may be thermal electric cooler (TEC) coupled to and/or including aheater/cooler 5404. In some embodiments, devices other than a TEC may beused to control the temperature such as “heat-only” devices and“cool-only” devices. At a first temperature, an optical source 5400 maygenerate a beam of light with a spectrum (in other words with adistribution curve of wavelengths). In some cases, the distributioncurve of the wavelengths may not be aligned with the spectrum of theoutput of the spectrally-equalizing amplifier. In this case, the outputamplified bandwidth may not reach its maximum capacity and thus thecoherence length not being set to its optimally shortest length. In somecases, the maximum output bandwidth of the amplifier may be desirable.In some cases, by changing the temperature of the optical source 5400such that the spectrum generated by the optical source aligns with thespectrum of the output of the spectrally-equalizing amplifier, both themaximum capacity of the output amplified bandwidth and the shortestpossible coherence length can be reached, which may improve signalquality. In some embodiments, the coherence length may be shortened to arange of approximately 10-15 microns. Aligning the wavelengths byadjusting the temperature may be described further with respect to FIGS.56A and B. Additionally, aligning the wavelengths may improve theamplifier's ability to perform spectral equalization on the beam oflight received from the optical source 5400 and/or the modulator. Insome embodiments, the amplifier may be an EDFA. In some embodiments, theamplifier may be a Raman amplifier, similarly to what was described withrespect to FIG. 52 .

In some embodiments, a feedback mechanism may be implemented to provideinformation to the system as to what the current temperature of theoptical source 5400 is set at (e.g., based on a sensor trackingtemperature readings such as the thermometer 5406) as well asinformation (e.g., from sensors 5508) related to a performance of thesystem. For instance, sensors 5508 may report data regarding howeffective was the most recent set of transmissions (which may usetesting techniques described with respect to FIG. 44 ), what are thecurrent conditions (e.g., weather, visibility, etc.), any drift, and thelike. In this manner, the system may adjust the temperature adjustmentto compensation for changes, or adjust other system parameters (e.g.,encoding technique, gain equalizer (if actively controlled), and thelike). Additionally, the system may call an operation that defines anexpected curve on how to drive a wavelength of a beam generated by theoptical source 5400 given a current temperature reading of the source5400.

In some embodiments, the temperature controller 5402 may include a PIDcontroller may be implemented to determine how to adjust thetemperature. In doing so, the PID controller may use one or more of thefollowing: signals proportional to the temperature difference, integralof the temperature difference over time, the derivative of thetemperature over time, and/or the like.

In some cases, the feedback information indicative of a performance ofthe system may be sent and considered by the system when determining howto adjust the temperature. The system may make such adjustments inreal-time (in other words, satisfying pre-configured real-timerequirements). The performance may be related to signal quality and/orother performance metrics related to signal quality. For example, thefeedback information may convey the BER at a current temperaturereading. The system may determine that driving the wavelength of thesource in a certain direction (increasing it or decreasing it) by eitherheating or cooling may lower the BER and may send an instruction to thetemperature controller 5202 according to such determination. Evaluatingthe signal quality and/or other performance metrics may take place on asideband linked to the system. For example, the system may transmit asignal at a higher than needed bit rate and slice out the extra-bits inreal-time in order to perform an evaluation such as a health exchange onthe extra-bits via a sideband. In some examples, the system may performdithering (pointing and centering) to determine how to adjust thetemperature to improve signal quality. For example, dithering mayinvolve the system making sequential small adjustments to thetemperature in both directions to perform a “perturb-and-observe” teston whether the BER improved or worsened. If it worsened, the system maydetermine to change the temperature in the direction opposite of whatwas tested. If it improved, the system may determine to change thetemperature in the same direction as to what was tested. In other words,the system may detect the control point at which the BER (or anotherperformance metric) is minimal/improved and set the temperatureaccordingly.

In some embodiments, feedback information indicative of an atmosphericcondition may be sent to and considered by the system when determininghow to adjust the temperature. The feedback information may be generatedusing a sensor linked to the system. In some examples, the feedbackinformation may indicate the presence of higher than usual atmosphericturbulence (clear air scintillation). Based on such information, thesystem may determine that the output spectrum of the amplifier should beset to the maximum limit in terms of having the shortest coherencelength possible and the system may adjust the temperature accordingly.

In some cases, a temperature controller 5402 on a first end (a transmitend) and a temperature controller 5402 on a second end (a receive end)of a bi-directional communication link may drive distribution curves todifferent ranges of wavelengths to thereby provide co-transmission alonga same region of space. The systems on each end may communicate todivide respective wavelength portions and the temperature controllers5402 may drive respective temperatures of their respective opticalsources to output spectrum in the respective wavelength potions.

FIG. 55 shows an exemplary system with multiple sources used incombination to optically transmit data through a refractive medium. Thesystem may include multiple optical sources (first source 5508, secondsource 5510, and third source 5512 up to and including an Nth source5514) independently linked to respective temperature controllers (firsttemperature controller 5500, second temperature controller 5502, andthird temperature controller 5504 up to and including nth temperaturecontroller N 5506), a combiner 5516, and/or an amplifier 5518. Thesystem may include additional components and/or features described withrespect to FIG. 54 .

The sources may be independent of one another and may be set to generatebeams with different wavelengths and different data modulation ratesthat can be optically combined to generate an output from the amplifier5518 with a broadened bandwidth and shorter coherence length. Forexample, as shown and described in FIG. 56A, the bottom graph may add ahump or in other words a potential distribution of wavelengths for eachadditional independent source. Having more potential distributions tochoose from may lead to finding a combination of sources set todifferent wavelengths (via independent temperature tuning) that resultsin a broader (or broadest) output from the amplifier 5518.

In some embodiments, each source may be linked to a respectivetemperature controller, which may be used to configure the sources togenerate beams with different wavelengths. In some embodiments, thesources and/or temperature controllers may be in communication with oneanother such that adjusting the wavelengths is based at least partiallyon the current wavelengths and/or temperature readings associated withthe other sources.

The combiner 5516 may receive the individual beam generated and sentfrom respective optical sources. In some embodiments, the combiner 5516may be a passive combiner. For example, if the number of sources is 4,the combiner may be a 1×4 power splitter.

The combiner 5516 may feed the multiple beams of light into theamplifier 5518. The amplifier 5518 may be a spectrally-equalizingamplifier. The amplifier may include one or more components and/orfeatures described with respect to FIGS. 52 and 54 .

FIG. 56A shows exemplary diagrams 5600 demonstrating how to align thewavelengths of the output of the source and the output of the amplifierto achieve a maximally broad output bandwidth.

The top diagram may depict an output spectrum (distribution curve ofwavelengths) of an amplified output that has not been equalized (similarto the graph on the left shown in FIG. 52 ). The middle diagram maydepict an output spectrum (distribution curve of wavelengths) of anamplified output that has been equalized using a spectrally-equalizingamplifier (similar to the graph on the right shown in FIG. 52 ). In eachgraph, the dashed line may represent 50% of the bandwidth, respectively(for example, 50% of 3 dB). The bottom diagram may depict the outputspectrum (power density relative to wavelengths) of the source at 3different temperature settings. It should be noted that since thecurrent applied by the system remains constant for the varyingtemperature settings, the shapes of the output spectrums at differenttemperatures (distributions of wavelengths) may remain more or lessidentical. This may allow the system to determine a temperature settingby choosing the setting that results in the center wavelength of thespectrum of the output beam being aligned with the center wavelength ofthe amplified output spectrum (described above). Considering the threedifferent output spectrum options depicted in the bottom graph, thesystem may determine to set the temperature to the setting that producesthe middle output spectrum as it would lead to the broadest amplifiedoutput since the spectrum of the optical source would produce power forthe greatest range of wavelengths that align with the bandwidth. Asdescribed with respect to FIG. 55 , adding multiple sources and/ormultiple temperature controllers may allow for more distribution curvesto be considered; thereby, the system may configure the optical sourcesto generate a beam with an output spectrum that results in a maximumamplified output bandwidth, or maximum amplified output power.

FIG. 56B shows exemplary diagrams (graph on left 5602 and graph on right5604) demonstrating a system configured to tune a temperature of anoptical source to account for a changed state of the amplifier.

In some embodiments, a feedback mechanism may send informationindicative of a condition of the amplifier. For example, the informationmay convey that the amplifier is performing spectral equalization on thereceived beam of light differently than in the past, which is resultingin an amplified output spectrum that is not flat enough to produce asignal with a sufficiently short coherence length. The graph on theright 5604 may show an output spectrum that over time has drifted and isbeginning to resemble the output spectrum of the output from anamplifier that does not perform gain-equalizing (graph on the left5602). The decline in performance may be due to deterioration such asshort-term, medium-term or long-term drift. In such a case, thetemperature of the source may be adjusted to account for thedeteriorated state of the amplifier such that the amplifier is able tooutput a spectrum that is sufficiently equalized, for example,sufficiently flattened. Using these system optimization techniques mayextend the service-life of the system.

FIG. 57 shows an exemplary method for tuning a temperature of an opticalsource to optically transmit data through a refractive medium. Themethod may start at step 5700, where the system may sense, using athermometer, a temperature of an optical source. For instance, thethermometer may report a current temperature of the optical source (or,indirectly, a heat sink attached or adjacent the optical source).

At step 5702, the system may determine, using a temperature controller,a temperature adjustment based on the temperature or other data (e.g.,from sensors). In some cases, the temperature adjustment may beconfigured to modify a distribution curve of wavelengths of the beam oflight output by the optical source, as discussed herein.

At step 5704, the system may adjust, using a heater/cooler, thetemperature based on the temperature adjustment. For instance, thetemperature controller may transmit an instruction to the heater/coolerto raise or lower the temperature of the optical source, as discussedherein, in accordance with the determined temperature adjustment.

At step 5706, the system may, by an optical source, generate a beam oflight, and transmit, detect and determine system parameters. Forinstance, the optical source may be a SLED, as discussed herein. Thesystem may also collect data from sensors (e.g., weather, alignment orpower drift, and the like) and determine a new temperature adjustment orother system parameters. Other system parameters may include encodingtechnique, adjustments to gain equalizer (if actively controlled), andthe like, as discussed herein. The system may then return to step 5700to repeat the method.

Impulsive Detection

FIGS. 58A, 58B, and 59 depict features for optically transmitting datathrough a variably refractive medium using pulses short enough wheredata may be extracted using impulsive detection. The features of FIGS.58A, 58B, and 59 may apply or use any feature of FIGS. 35A-51 and 52-57.

FIG. 58A shows an exemplary system for optically transmitting datathrough a variably refractive medium using pulses short enough wheredata may be extracted from a pulse at a rate less than a detectionresponse time. The system may include an optical source 5800, a timeslicing modulator 5802, a pre-amplifier 5804, a data modulator 5806,and/or an amplifier 5808. It should be noted that either or bothmodulators may be altered to include additional or alternative features.In some embodiments, the modulator(s) may not be included in the system.

An optical source 5800 may be configured to generate a beam of light.The optical source 5800 may be similar to and/or may havecharacteristics of the optical sources described with respect to FIGS.52 and 54 . In some embodiments, the optical source 5800 may beconfigured to send a beam at a duty factor (for example, 50%) highenough that when the beam is chopped into return-to-zero (RZ) pulses(described further herein), the pulses have enough average power to notbe disturbed (materially disturbed) by noise (ASE noise) produced by theamplifier 5808. In such a case, a pre-amplifier 5804 may not be neededin the system as the average power of the pulses would already besufficient.

A time slicing modulator 5802 may slice the beam received from theoptical source 5800 into one or more pulses. Slicing the beam mayinvolve narrow-time slicing techniques. For example, the time slicingmodulator 5802 may be a short pulse generator such as an electrical combdrive. The pulses may be sliced short enough such that transmitted data(similar to the transmitted data described with respect to FIG. 52 ) maybe extracted from a pulse at a rate less than a detection response timethreshold. The threshold may be set to be at least a factor of 1.25,1.5, 2, 2.5, and the like below the detector response time. Toillustrate using an example, a detector with a 3 dB bandwidth may have aresponse time of 400 picoseconds. In such a case, the threshold may beset to at least 200 picoseconds and the rate at which data must beextracted from a pulse in order to be considered in an RZ state must bebelow 200 picoseconds. Another way to think about the slicing mechanismis that the pulses may be sliced to achieve an RZ state (when datacannot be extracted from the pulses at a rate less than the detectionresponse time threshold, it may be referred to as being in anon-return-to-zero (NRZ) state). While in an RZ state, one or moreimpulsive coding benefits may be realized by the system such asincreased detection efficiency, increased link margin, detector hitswith increased power, and/or the like.

A pre-amplifier 5804 may be included in the system to receive one ormore pulses. Due to the time-slicing performed by the time slicingmodulator 5802, the average power of the pulses may be significantlyless than the average power of the original beam generated by theoptical source 5800. For example, the average power of the pulses may beless than one of 2.5%, 5%, 10%, 20%, 30%, 40%, 50%, etc. of what theaverage power of the beam was. Power distribution curves for theoriginal beam of light as well as the chopped pulses may be depicted anddescribed with respect to FIG. 58B. The pre-amplifier 5804 may beconfigured to add power to the one or more pulses such that noise (ASEnoise) generated from the amplifier 5808 will not dominate the noise(ASE noise) of the amplified output pulses. In some embodiments, thepre-amplifier 5804 may increase the average power of the pulses to theaverage power of the original beam was, for example, which may be in therange of a few hundred mW. In some embodiments, the preamplifier 5804may include CW gating light source(s), while in others it may notinclude CW gating light source(s). In some embodiments, the preamplifier5804 may be configured to perform gain-flattening techniques describedwith respect to FIGS. 52-57 . In some embodiments, the pre-amplifier5804 may perform gain-flattening on the pulses in order for thepre-amplified output spectrum to be spectrally broad and follow adistribution curve similar to the amplified output spectrum. Otherwisein some cases, a pre-amplifier 5804 without gain-flattening may output aspectrum that is so narrow that the amplifier 5808 may be unable toamplify a meaningful portion of the pre-amplified spectrum. In someembodiments, the pre-amplifier 5804 may be associated with a broadbandwidth.

A data modulator 5806 may receive the pulses from the pre-amplifier 5804and encode data on the pulses. The data modulator 5806 may be similar toand/or may perform similar keying operations and/or other encodingtechniques as the modulators described with respect to FIGS. 44, 52, and54 .

The amplifier 5808 may receive the plurality of pulses and amplify andfilter the pulses. In some embodiments, the amplifier 5808 may be aspectrally-equalizing amplifier and may perform spectral equalization,for example, gain-flattening, on a gain applied to the pulses (which isdescribed further with respect to FIGS. 52-56 ). As mentioned above, theamplifier 5808 may produce its own noise (ASE noise) independent of thereceived pulses. In cases where the pulses are not processed by apre-amplifier 5804, this noise may disturb the quality of the pulses dueto the average power of the pulses being too low (caused by slicing thebeam). As described above, a pre-amplifier 5804 or any other device thatcan amplify, or increase the power of the pulses may be used to preventsuch disturbances. The amplifier 5808 may additionally add more power tothe pulses, for examples, may increase the pulses from a range of arounda few hundred mW to a range of around 2 Watts, in order to traverse theoptical communication distance.

FIG. 58B shows exemplary diagrams of power distributions of the beamgenerated by the optical source and of the chopped pulses.

The top graph 5810 may show the power density over time of the beamoutputted by the optical source which, for example, may be CW. Thebottom graph 5812 may show power density over time of the original beamsliced into sliced pulses. In some embodiments, slicing the beam mayinvolve removing at least 90% (e.g., 90%, 95%, or 97.5%, and the like)of a bit duration from the original beam for a given time window. Whencomparing the two graphs, bits encoded on the beam (top graph 5810) mayhave a longer bit duration (demonstrated by the longer time intervals atwhich a bit is present for a given time window, which is shown via thepositive intensity reading) than bits encoded on the pulses (bottomgraph 5812). This may lead to the conclusion that chopping the beamresults in a decreased average power associated with the pulses. Forexample, using the example of removing 90% of the bit duration mentionedabove, a scenario may exist where the top graph 5810 may depict 800picosecond long bits while the bottom graph 5812 may depict 80picosecond long bits. Using pulses with such a short duration shouldprovide the impulsive coding benefits described with respect to FIG.58A. Additionally, in such a case, the average power of the pulses maybe a factor of 10 less than the average power of the original beam(which would naturally follow since by removing 90% of the bit duration,10% would remain). In some embodiments, this factor, for example, in theabove scenario a factor of 10, may be the gain applied to the pulses bythe pre-amplifier in order to restore the average power (in other words,the gain may be the ratio of the average power of the original beam overthe average power the chopped pulses). For example, for a factor of 20,the bit duration of the chopped pulses may be 40 picoseconds; for afactor of 40, it may be 20 picoseconds. The factors may range from lessthan 10, between 10 and 40, or more than 40.

FIG. 59 shows an exemplary method for optically transmitting datathrough a variably refractive medium using pulses short enough wheredata can be extracted from a pulse at a rate less than a detectionresponse time. The method may start at step 5900, where the system maygenerate, by an optical source, a beam of light. In some cases, theoptical source may include a waveguide that amplifies emitted light. Forinstance, the optical source may be a SLED, a mode-locked laser, and thelike, as discussed herein.

At step 5902, the system may slice, by a first modulator, the beam oflight into a plurality of pulses. In some cases, the first modulator mayremove at least 90% of the beam of light (i.e., block it or otherwiseabsorb it), as discussed herein.

At step 5904, the system may amplify, by a pre-amplifier, the pluralityof pulses to produce pre-amplified plurality of pulses. For instance,the pre-amplified plurality of pulses may have an average power thatcorresponds to an average power of the beam of light. In some cases, thepre-amplifier is a spectrally-equalizing amplifier (e.g., a gainflattened amplifier), as discussed herein. The pre-amplifier may apply again factor of at least 10, but the gain factor may be anywhere between10 and several thousand. In some cases, the pre-amplifier may alsospectrally-equalize the power amplification.

At step 5906, the system may encode, by a second modulator, data on thepre-amplified plurality of pulses to produce an encoded plurality ofpulses. For instance, the modulator may receive a data signal and on-offkey (e.g., return to zero) the pre-amplified plurality of pulses inaccordance with the digital signal and time slots, as discussed herein.

At step 5908, the system may amplify and filter, by aspectrally-equalizing amplifier, the encoded plurality of pulses toproduce a filtered plurality of pulses. The spectrally-equalizingamplifier may spectrally equalize a gain applied to the encodedplurality of pulses, as discussed herein.

At step 5910, the system may transmit the filtered plurality of pulsesthrough a variably refractive medium to a detector having aphotoreceiver. For instance, the system may route the filtered pluralityof pulses to a telescope that transmits the filtered plurality ofpulses, e.g., to a detector, as discussed herein.

Examples

Exemplary embodiments of the systems and methods disclosed herein aredescribed in the numbered paragraphs below.

A1. An optical communication system for optically transmitting datathrough a variably refractive medium, the optical communication systemcomprising:

-   -   an optical source configured to generate a beam of light, the        optical source comprising a waveguide that amplifies emitted        light;    -   a modulator configured to encode data on the beam of light to        produce an encoded beam of light; and    -   a spectrally-equalizing amplifier configured to receive the        encoded beam of light from the modulator and both amplify and        filter the encoded beam of light to produce a filtered beam of        light, wherein the spectrally-equalizing amplifier spectrally        equalizes a gain applied to the encoded beam of light,    -   wherein the optical communication system is configured to        transmit the filtered beam of light through a variably        refractive medium to a detector having a photoreceiver, wherein        the photoreceiver is configured to extract the data from the        filtered beam of light.        A2. The optical communication system of A1, wherein the optical        communication system has a measured bit error rate of less than        one in one million over a free space optical communication        distance for a measurement period of at least sixty seconds.        A3. The optical communication system of any of A1-A2, wherein        the spectrally-equalizing amplifier is a gain-flattened        amplifier that flattens the gain applied to the encoded beam of        light.        A4. The optical communication system of A3, wherein the        gain-flattened amplifier is a fiber amplifier comprising at        least a core and a cladding surrounding the core, wherein the        cladding comprises a transition metal ion compound, and the core        of the gain-flattened amplifier is configured to receive the        encoded beam of light from the modulator and both amplify and        filter the encoded beam of light to produce the filtered beam of        light.        A5. The optical communication system of any of A1-A4, wherein,        based on spectrally equalizing the gain applied to the encoded        beam of light, amplified output wavelengths within a range are        made to have substantively equal spectral power density.        A6. The optical communication system of any of A1-A5, wherein        the optical source is a superluminescent diode (SLED), and the        spectrally-equalizing amplifier is a nonlinear filter that        amplifies the encoded beam of light and reduces high frequency        noise.        A7. The optical communication system of any of A1-A6, wherein:    -   the filtered beam of light comprises a plurality of pulses        including at least a first pulse that is transmitted over an        optical communication distance to the photoreceiver;    -   as the first pulse traverses over the optical communication        distance, photons, of the first pulse, travel along a plurality        of ray paths having different lengths to the photoreceiver;    -   the photons of the first pulse of light arrive at the        photoreceiver according to a temporal distribution curve that        depends, at least in part, on a duration of the first pulse and        the different lengths of the plurality of ray paths taken by the        photons in the first pulse to the photoreceiver; and    -   a full width at half maximum (FWHM) value of the temporal        distribution curve is at least three times as large as a        coherence time value equal to a coherence length of the first        pulse divided by a speed of light through the variably        refractive medium.        A8. The optical communication system of any of A1-A7, wherein        the spectrally-equalizing amplifier includes one or more of:        long-period fiber grating, a MEMs equalizer, or a thin film        coating.        A9. The optical communication system of any of A1-A8, wherein        the spectrally-equalizing amplifier is a Raman amplifier.        A10. The optical communication system of any of A1-A9, wherein        the gain applied to the encoded beam of light is flattened to        produce a distribution curve of wavelengths associated with the        filtered beam, wherein the distribution curve has a variance        less than ±1 dB over a given range of wavelengths.        A11. A method for optically transmitting data through a variably        refractive medium, the method comprising:    -   generating, by an optical source, a beam of light, the optical        source comprising a waveguide that amplifies emitted light;    -   encoding, by a modulator, data on the beam of light to produce        an encoded beam of light;    -   amplifying and filtering, by a spectrally-equalizing amplifier,        the encoded beam of light to produce a filtered beam of light,        wherein the spectrally-equalizing amplifier spectrally equalizes        a gain applied to the encoded beam of light; and    -   transmitting the filtered beam of light through a variably        refractive medium to a detector having a photoreceiver, wherein        the photoreceiver is configured to extract the data from the        filtered beam of light.        A12. The method of A11, wherein the filtered beam of light has a        measured bit error rate of less than one in one million over a        free space optical communication distance for a measurement        period of at least sixty seconds.        A13. The method of any of A11-A12, wherein the        spectrally-equalizing amplifier is a gain-flattened amplifier        that flattens the gain applied to the encoded beam of light.        A14. The method of A13, wherein the gain-flattened amplifier is        a fiber amplifier comprising at least a core and a cladding        surrounding the core, wherein the cladding comprises a        transition metal ion compound and the core of the gain-flattened        amplifier is configured to receive the encoded beam of light        from the modulator and both amplify and filter the encoded beam        of light to produce the filtered beam of light.        A15. The method of any of A11-A14, wherein, based on spectrally        equalizing the gain applied to the encoded beam of light,        amplified output wavelengths within a range are made to have        substantively equal spectral power density.        A16. The method of any of A11-A15, wherein the optical source is        a superluminescent diode (SLED), and the spectrally-equalizing        amplifier is a nonlinear filter that amplifies the encoded beam        of light and reduces high frequency noise.        A17. The method of any of A11-A16, wherein:    -   the filtered beam of light comprises a plurality of pulses        including at least a first pulse that is transmitted over an        optical communication distance to the photoreceiver;    -   as the first pulse traverses over the optical communication        distance, photons, of the first pulse, travel along a plurality        of ray paths having different lengths to the photoreceiver;    -   the photons of the first pulse of light arrive at the        photoreceiver according to a temporal distribution curve that        depends, at least in part, on a duration of the first pulse and        the different lengths of the plurality of ray paths taken by the        photons in the first pulse to the photoreceiver; and    -   a full width at half maximum (FWHM) value of the temporal        distribution curve is at least three times as large as a        coherence time value equal to a coherence length of the first        pulse divided by a speed of light through the variably        refractive medium.        A18. The method of any of A11-A17, wherein the        spectrally-equalizing amplifier includes one or more of:        long-period fiber grating, a MEMs equalizer, or a thin film        coating.        A19. The method of any of A11-A18, wherein the        spectrally-equalizing amplifier is a Raman amplifier.        A20. The method of any of A11-A19, wherein the gain applied to        the encoded beam of light is flattened to produce a distribution        curve of wavelengths associated with the filtered beam, wherein        the distribution curve has a variance less than 1 dB over a        given range of wavelengths.        B1. An optical communication system for optically transmitting        data through a variably refractive medium, the optical        communication system comprising:    -   an optical source configured to generate a beam of light, the        optical source comprising a waveguide that amplifies emitted        light;    -   a modulator configured to encode data on the beam of light to        produce an encoded beam of light;    -   a spectrally-equalizing amplifier configured to receive the        encoded beam of light from the modulator and both amplify and        filter the encoded beam of light to produce a filtered beam of        light, wherein the spectrally-equalizing amplifier spectrally        equalizes a gain applied to the encoded beam of light;    -   wherein the optical communication system is configured to        transmit the filtered beam of light through a variably        refractive medium to a detector having a photoreceiver, wherein        the photoreceiver is configured to extract the data from the        filtered beam of light; and    -   a temperature controller having a thermometer and a        heater/cooler, wherein the temperature controller is configured        to:        -   sense, using the thermometer, a temperature of the optical            source;        -   determine a temperature adjustment based on the temperature,            wherein the temperature adjustment is configured to modify a            distribution curve of wavelengths of the beam of light; and        -   adjust, using the heater/cooler, the temperature based on            the temperature adjustment.            B2. The optical communication system of B1, wherein the            spectrally-equalizing amplifier is a gain-flattened            amplifier that flattens the gain applied to the encoded beam            of light.            B3. The optical communication system of any of B1-B2,            wherein modifying the distribution curve of wavelengths of            the beam of light includes aligning a center of the            distribution curve with the gain applied by the            spectrally-equalizing amplifier.            B4. The optical communication system of any of B1-B3,            wherein the temperature controller includes a            proportional-integral-derivative controller (PID            controller), wherein the PID controller determines the            temperature adjustment.            B5. The optical communication system of any of B1-B4,            wherein the optical source is a superluminescent diode            (SLED), and the spectrally-equalizing amplifier comprises a            nonlinear filter that amplifies the encoded beam of light            and reduces high frequency noise.            B6. The optical communication system of any of B1-B5,            further comprising:    -   a sensor configured to provide information related to a        deterioration condition of the spectrally-equalizing amplifier        to the temperature controller,    -   wherein the temperature controller is configured to determine        the temperature adjustment so as to modify the distribution        curve of wavelengths of the beam of light to correct drifting        caused by the deterioration condition.        B7. The optical communication system of any of B1-B6, wherein        the optical source is a first optical source, the temperature        controller is a first temperature controller, the detector is        associated with a second optical source and a second temperature        controller, and    -   the first temperature controller and second temperature        controller are configured to, respectively, determine        temperature adjustments to modify distribution curves of first        and second optical sources such that the distribution curves are        different from each other.        B8. The optical communication system of any of B1-B7, wherein        the temperature controller is configured to monitor a bit error        rate of the photoreceiver for the filtered beam of light at        different temperature settings, and, at periodic iterations, the        temperature controller changes the temperature adjustment to a        different temperature adjustment to determine whether the        photoreceiver extracts the data at a lower bit error rate.        B9. The optical communication system of any of B1-B8, wherein        the optical source is a first optical source among a plurality        of optical sources, the temperature controller is a first        temperature controller among a plurality of temperature        controllers, and the plurality of temperature controllers are        configured to, respectively, determine temperature adjustments        to modify distribution curves of respective optical sources, and        the distribution curves are different from each other.        B10. The optical communication system of any of B1-B9, further        comprising:    -   a sensor configured to provide information related to an        atmospheric condition of the variably refractive medium to the        temperature controller,    -   wherein the temperature controller is configured to determine        the temperature adjustment so as to modify the distribution        curve of wavelengths of the beam of light to correspond to a        signal that propagates efficiently with respect to the        atmospheric condition.        B11. A method for optically transmitting data through a variably        refractive medium, the method comprising:    -   generating, by an optical source, a beam of light, wherein the        optical source includes a waveguide that amplifies emitted        light;    -   sensing, using a thermometer, a temperature of the optical        source;    -   determining, using a temperature controller, a temperature        adjustment based on the temperature, wherein the temperature        adjustment is configured to modify a distribution curve of        wavelengths of the beam of light;    -   adjusting, using a heater/cooler, the temperature based on the        temperature adjustment;    -   encoding, by a modulator, data on the beam of light to produce        an encoded beam of light;    -   amplifying and filtering, by a spectrally-equalizing amplifier,        the encoded beam of light to produce a filtered beam of light,        wherein the spectrally-equalizing amplifier spectrally equalizes        a gain applied to the encoded beam of light; and    -   transmitting the filtered beam of light through a variably        refractive medium to a detector having a photoreceiver, wherein        the photoreceiver is configured to extract the data from the        filtered beam of light.        B12. The method of B11, wherein the spectrally-equalizing        amplifier is a gain-flattened amplifier that flattens the gain        applied to the encoded beam of light.        B13. The method of any of B11-B12, wherein modifying the        distribution curve of wavelengths of the beam of light includes        aligning a center of the distribution curve with the gain        applied by the spectrally-equalizing amplifier.        B14. The method of any of B11-B13, wherein the temperature        controller includes a proportional-integral-derivative        controller (PID controller), wherein the PID controller        determines the temperature adjustment.        B15. The method of any of B11-B14, wherein the optical source is        a superluminescent diode (SLED), and the spectrally-equalizing        amplifier comprises a nonlinear filter that amplifies the        encoded beam of light and reduces high frequency noise.        B16. The method of any of B11-B15, further comprising:        receiving, at the temperature controller from a sensor,        information related to a deterioration condition of the        spectrally-equalizing amplifier, wherein the temperature        controller is configured to determine the temperature adjustment        so as to modify the distribution curve of wavelengths of the        beam of light to correct drifting caused by the deterioration        condition.        B17. The method of any of B11-B16, further comprising:        receiving, at the temperature controller from a sensor,        information related to an atmospheric condition of the variably        refractive medium, wherein the temperature controller is        configured to determine the temperature adjustment so as to        modify the distribution curve of wavelengths of the beam of        light to correspond to a signal that propagates efficiently with        respect to the atmospheric condition.        B18. The method of any of B11-B17, further comprising:    -   monitoring, by the temperature controller, a bit error ratre of        the photoreceiver for the filtered beam of light at different        temperature settings; and    -   at periodic iterations, changes, by the temperature controller,        the temperature adjustment to a different temperature adjustment        to determine whether the photoreceiver extracts the data at a        lower bit error rate.        B19. The method of any of B11-B18, wherein the optical source is        a first optical source among a plurality of optical sources, the        temperature controller is a first temperature controller among a        plurality of temperature controllers, and the method further        comprises:    -   determining, by the plurality of temperature controllers,        temperature adjustments to modify distribution curves of        respective optical sources, wherein the distribution curves are        different from each other.        B20. The method of any of B11-B19, wherein the optical source is        a first optical source, the temperature controller is a first        temperature controller, the detector is associated with a second        optical source and a second temperature controller, and the        method further comprises:    -   determining, by the first temperature controller and second        temperature controller, temperature adjustments to modify        distribution curves of first and second optical sources such        that the distribution curves are different from each other.        C1. An optical communication system for optically transmitting        data through a variably refractive medium, the optical        communication system comprising:    -   an optical source configured to generate a beam of light,        wherein the optical source includes a waveguide that amplifies        emitted light;    -   a first modulator configured to slice the beam of light into a        plurality of pulses;    -   a pre-amplifier configured to receive the plurality of pulses        from the first modulator and amplify the plurality of pulses to        produce pre-amplified plurality of pulses, wherein the        pre-amplified plurality of pulses has an average power that        corresponds to an average power of the beam of light;    -   a second modulator configured to encode data on the        pre-amplified plurality of pulses to produce an encoded        plurality of pulses; and    -   a spectrally-equalizing amplifier configured to receive the        encoded plurality of pulses and both amplify and filter the        encoded plurality of pulses to produce a filtered plurality of        pulses, wherein the spectrally-equalizing amplifier spectrally        equalizes a gain applied to the encoded plurality of pulses,    -   wherein the optical communication system is configured to        transmit the filtered plurality of pulses through a variably        refractive medium to a detector having a photoreceiver, wherein        the photoreceiver is configured to extract the data from the        filtered plurality of pulses at a rate less than a detection        response time threshold of the detector.        C2. The optical communication system of C1, wherein the        spectrally-equalizing amplifier is a gain-flattened amplifier        that flattens the gain applied to the encoded plurality of        pulses.        C3. The optical communication system of any of C1-C2, wherein        the optical communication system has a measured bit error rate        of less than one in one million over a free space optical        communication distance for a measurement period of at least        sixty seconds.        C4. The optical communication system of any of C1-C3, wherein        the spectrally-equalizing amplifier is a fiber amplifier        including at least a core and a cladding surrounding the core,        wherein the cladding includes a transition metal ion compound        and the core of the spectrally-equalizing amplifier is        configured to receive the encoded plurality of pulses and both        amplify and filter the encoded plurality of pulses to produce        the filtered plurality of pulses.        C5. The optical communication system of any of C1-C4, wherein        the pre-amplifier is a gain-flattened amplifier that flattens        the gain applied to the plurality of pulses.        C6. The optical communication system of any of C1-C5, wherein        the optical source is a superluminescent diode (SLED), and the        spectrally-equalizing amplifier includes a nonlinear filter that        amplifies the encoded plurality of pulses and reduces high        frequency noise.        C7. The optical communication system of any of C1-C6, wherein:    -   the filtered plurality of pulses includes at least a first pulse        that is transmitted over an optical communication distance to a        detector having a photoreceiver;    -   as the first pulse traverses over the optical communication        distance, photons, of the first pulse, travel along a plurality        of ray paths having different lengths to the photoreceiver;    -   the photons of the first pulse of light arrive at the        photoreceiver according to a temporal distribution curve that        depends, at least in part, on a duration of the first pulse and        the different lengths of the plurality of ray paths taken by the        photons in the first pulse to the photoreceiver; and    -   a full width at half maximum (FWHM) value of the temporal        distribution curve is at least three times as large as a        coherence time value equal to a coherence length of the first        pulse divided by a speed of light through the variably        refractive medium.        C8. The optical communication system of any of C1-C7, wherein        slicing the beam of light into the plurality of pulses includes        removing at least 90% of a bit duration for a given time window,        the plurality of pulses having an average power equal to or less        than 10% of the average power of the beam of light, and the        pre-amplifier is configured to amplify the plurality of pulses        by a gain of at least a factor of 10.        C9. The optical communication system of any of C1-C8, wherein        the first modulator performs a return-to-zero modulation.        C10. The optical communication system of any of C1-C9, wherein        slicing, pre-amplifying, encoding, and spectrally-equalizing the        beam of light causes the filtered plurality of pulses to be        impulsively detected by the photoreceiver.        C11. A method for optically transmitting data through a variably        refractive medium, the method comprising:    -   generating, by an optical source, a beam of light, wherein the        optical source includes a waveguide that amplifies emitted        light;    -   slicing, by a first modulator, the beam of light into a        plurality of pulses;    -   amplifying, by a pre-amplifier, the plurality of pulses to        produce pre-amplified plurality of pulses, wherein the        pre-amplified plurality of pulses has an average power that        corresponds to an average power of the beam of light;    -   encoding, by a second modulator, data on the pre-amplified        plurality of pulses to produce an encoded plurality of pulses;    -   amplifying and filtering, by a spectrally-equalizing amplifier,        the encoded plurality of pulses to produce a filtered plurality        of pulses, wherein the spectrally-equalizing amplifier        spectrally equalizes a gain applied to the encoded plurality of        pulses; and    -   transmitting the filtered plurality of pulses through a variably        refractive medium to a detector having a photoreceiver, wherein        the photoreceiver is configured to extract the data from the        filtered plurality of pulses at a rate less than a detection        response time threshold of the detector.        C12. The method of C11, wherein the spectrally-equalizing        amplifier is a gain-flattened amplifier that flattens the gain        applied to the encoded plurality of pulses.        C13. The method of any of C11-C12, wherein the encoded plurality        of pulses has a measured bit error rate of less than one in one        million over a free space optical communication distance for a        measurement period of at least sixty seconds.        C14. The method of any of C11-C13, wherein the        spectrally-equalizing amplifier is a fiber amplifier including        at least a core and a cladding surrounding the core, wherein the        cladding includes a transition metal ion compound, and the core        of the spectrally-equalizing amplifier is configured to receive        the encoded plurality of pulses and both amplify and filter the        encoded plurality of pulses to produce the filtered plurality of        pulses.        C15. The method of any of C11-C14, wherein the pre-amplifier is        a gain-flattened amplifier that flattens the gain applied to the        plurality of pulses.        C16. The method of any of C11-C15, wherein the optical source is        a superluminescent diode (SLED), and the spectrally-equalizing        amplifier includes a nonlinear filter that amplifies the encoded        plurality of pulses and reduces high frequency noise.        C17. The method of any of C11-C16, wherein:    -   the filtered plurality of pulses includes at least a first pulse        that is transmitted over an optical communication distance to a        detector having a photoreceiver;    -   as the first pulse traverses over the optical communication        distance, photons, of the first pulse, travel along a plurality        of ray paths having different lengths to the photoreceiver;    -   the photons of the first pulse of light arrive at the        photoreceiver according to a temporal distribution curve that        depends, at least in part, on a duration of the first pulse and        the different lengths of the plurality of ray paths taken by the        photons in the first pulse to the photoreceiver; and    -   a full width at half maximum (FWHM) value of the temporal        distribution curve is at least three times as large as a        coherence time value equal to a coherence length of the first        pulse divided by a speed of light through the variably        refractive medium.        C18. The method of any of C11-C17, wherein slicing the beam of        light into the plurality of pulses includes removing at least        90% of a bit duration for a given time window, the plurality of        pulses having an average power equal to or less than 10% of the        average power of the beam of light, and the pre-amplifier is        configured to amplify the plurality of pulses by a gain of at        least a factor of 10.        C19. The method of any of C11-C18, wherein the first modulator        performs a return-to-zero modulation.        C20. The method of any of C11-C19, wherein slicing,        pre-amplifying, encoding, and spectrally-equalizing the beam of        light causes the filtered plurality of pulses to be impulsively        detected by the photoreceiver.

While the subject matter of this disclosure has been described and shownin considerable detail with reference to certain illustrativeembodiments, including various combinations and sub-combinations offeatures, those skilled in the art will readily appreciate otherembodiments and variations and modifications thereof as encompassedwithin the scope of the present disclosure. Moreover, the descriptionsof such embodiments, combinations, and sub-combinations are not intendedto convey that the claimed subject matter requires features orcombinations of features other than those expressly recited in theclaims. Accordingly, the scope of this disclosure is intended to includeall modifications and variations encompassed within the spirit and scopeof the following appended claims.

The invention claimed is:
 1. An optical communication system foroptically transmitting data through a variably refractive medium, theoptical communication system comprising: an optical source configured togenerate a beam of light, wherein the optical source includes awaveguide that amplifies emitted light; a first modulator configured toslice the beam of light into a plurality of pulses; a pre-amplifierconfigured to receive the plurality of pulses from the first modulatorand amplify the plurality of pulses to produce pre-amplified pluralityof pulses, wherein the pre-amplified plurality of pulses has an averagepower that corresponds to an average power of the beam of light; asecond modulator configured to encode data on the pre-amplifiedplurality of pulses to produce an encoded plurality of pulses; and aspectrally-equalizing amplifier configured to receive the encodedplurality of pulses and both amplify and filter the encoded plurality ofpulses to produce a filtered plurality of pulses, wherein thespectrally-equalizing amplifier spectrally equalizes a gain applied tothe encoded plurality of pulses, wherein the optical communicationsystem is configured to transmit the filtered plurality of pulsesthrough a variably refractive medium to a detector having aphotoreceiver, wherein the photoreceiver is configured to extract thedata from the filtered plurality of pulses at a rate less than adetection response time threshold of the detector.
 2. The opticalcommunication system of claim 1, wherein the spectrally-equalizingamplifier is a gain-flattened amplifier that flattens the gain appliedto the encoded plurality of pulses.
 3. The optical communication systemof claim 1, wherein the optical communication system has a measured biterror rate of less than one in one million over a free space opticalcommunication distance for a measurement period of at least sixtyseconds.
 4. The optical communication system of claim 1, wherein thespectrally-equalizing amplifier is a fiber amplifier including at leasta core and a cladding surrounding the core, wherein the claddingincludes a transition metal ion compound and the core of thespectrally-equalizing amplifier is configured to receive the encodedplurality of pulses and both amplify and filter the encoded plurality ofpulses to produce the filtered plurality of pulses.
 5. The opticalcommunication system of claim 1, wherein the pre-amplifier is again-flattened amplifier that flattens the gain applied to the pluralityof pulses.
 6. The optical communication system of claim 1, wherein theoptical source is a superluminescent diode (SLED), and thespectrally-equalizing amplifier includes a nonlinear filter thatamplifies the encoded plurality of pulses and reduces high frequencynoise.
 7. The optical communication system of claim 1, wherein: thefiltered plurality of pulses includes at least a first pulse that istransmitted over an optical communication distance to a detector havinga photoreceiver; as the first pulse traverses over the opticalcommunication distance, photons, of the first pulse, travel along aplurality of ray paths having different lengths to the photoreceiver;the photons of the first pulse of light arrive at the photoreceiveraccording to a temporal distribution curve that depends, at least inpart, on a duration of the first pulse and the different lengths of theplurality of ray paths taken by the photons in the first pulse to thephotoreceiver; and a full width at half maximum (FWHM) value of thetemporal distribution curve is at least three times as large as acoherence time value equal to a coherence length of the first pulsedivided by a speed of light through the variably refractive medium. 8.The optical communication system of claim 1, wherein slicing the beam oflight into the plurality of pulses includes removing at least 90% of abit duration for a given time window, the plurality of pulses having anaverage power equal to or less than 10% of the average power of the beamof light, and the pre-amplifier is configured to amplify the pluralityof pulses by a gain of at least a factor of
 10. 9. The opticalcommunication system of claim 1, wherein the first modulator performs areturn-to-zero modulation.
 10. The optical communication system of claim1, wherein slicing, pre-amplifying, encoding, and spectrally-equalizingthe beam of light causes the filtered plurality of pulses to beimpulsively detected by the photoreceiver.
 11. A method for opticallytransmitting data through a variably refractive medium, the methodcomprising: generating, by an optical source, a beam of light, whereinthe optical source includes a waveguide that amplifies emitted light;slicing, by a first modulator, the beam of light into a plurality ofpulses; amplifying, by a pre-amplifier, the plurality of pulses toproduce pre-amplified plurality of pulses, wherein the pre-amplifiedplurality of pulses has an average power that corresponds to an averagepower of the beam of light; encoding, by a second modulator, data on thepre-amplified plurality of pulses to produce an encoded plurality ofpulses; amplifying and filtering, by a spectrally-equalizing amplifier,the encoded plurality of pulses to produce a filtered plurality ofpulses, wherein the spectrally-equalizing amplifier spectrally equalizesa gain applied to the encoded plurality of pulses; and transmitting thefiltered plurality of pulses through a variably refractive medium to adetector having a photoreceiver, wherein the photoreceiver is configuredto extract the data from the filtered plurality of pulses at a rate lessthan a detection response time threshold of the detector.
 12. The methodof claim 11, wherein the spectrally-equalizing amplifier is again-flattened amplifier that flattens the gain applied to the encodedplurality of pulses.
 13. The method of claim 11, wherein the encodedplurality of pulses has a measured bit error rate of less than one inone million over a free space optical communication distance for ameasurement period of at least sixty seconds.
 14. The method of claim11, wherein the spectrally-equalizing amplifier is a fiber amplifierincluding at least a core and a cladding surrounding the core, whereinthe cladding includes a transition metal ion compound, and the core ofthe spectrally-equalizing amplifier is configured to receive the encodedplurality of pulses and both amplify and filter the encoded plurality ofpulses to produce the filtered plurality of pulses.
 15. The method ofclaim 11, wherein the pre-amplifier is a gain-flattened amplifier thatflattens the gain applied to the plurality of pulses.
 16. The method ofclaim 11, wherein the optical source is a superluminescent diode (SLED),and the spectrally-equalizing amplifier includes a nonlinear filter thatamplifies the encoded plurality of pulses and reduces high frequencynoise.
 17. The method of claim 11, wherein: the filtered plurality ofpulses includes at least a first pulse that is transmitted over anoptical communication distance to a detector having a photoreceiver; asthe first pulse traverses over the optical communication distance,photons, of the first pulse, travel along a plurality of ray pathshaving different lengths to the photoreceiver; the photons of the firstpulse of light arrive at the photoreceiver according to a temporaldistribution curve that depends, at least in part, on a duration of thefirst pulse and the different lengths of the plurality of ray pathstaken by the photons in the first pulse to the photoreceiver; and a fullwidth at half maximum (FWHM) value of the temporal distribution curve isat least three times as large as a coherence time value equal to acoherence length of the first pulse divided by a speed of light throughthe variably refractive medium.
 18. The method of claim 11, whereinslicing the beam of light into the plurality of pulses includes removingat least 90% of a bit duration for a given time window, the plurality ofpulses having an average power equal to or less than 10% of the averagepower of the beam of light, and the pre-amplifier is configured toamplify the plurality of pulses by a gain of at least a factor of 10.19. The method of claim 11, wherein the first modulator performs areturn-to-zero modulation.
 20. The method of claim 11, wherein slicing,pre-amplifying, encoding, and spectrally-equalizing the beam of lightcauses the filtered plurality of pulses to be impulsively detected bythe photoreceiver.